Control of n-(phosphonomethyl)iminodiacetic acid conversion manufacture of glyphosate

ABSTRACT

This invention relates to the preparation of N-(phosphonomethyl)glycine (“glyphosate”) from N-(phosphonomethyl)iminodiacetic acid (“PMIDA”), and more particularly to methods for control of the conversion of PMIDA, for the identification of reaction end points relating to PMIDA conversion and the preparation of glyphosate products having controlled PMIDA content.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/198,188 filed Jun. 30, 2016. U.S. application Ser. No. 15/198,188 isa continuation of U.S. application Ser. No. 12/884,289, filed Sep. 17,2010, now issued as U.S. Pat. No. 9,409,935. U.S. application Ser. No.12/884,289 is a continuation of U.S. application Ser. No. 11/910,146,filed on Jun. 20, 2008, now issued as U.S. Pat. No. 7,799,571. U.S.application Ser. No. 11/910,146 is a U.S. National Stage Application ofPCT/US2006/012214, filed Apr. 3, 2006, and claiming priority to U.S.Provisional Application No. 60/667,783, filed Apr. 1, 2005.

BACKGROUND OF THE INVENTION

This invention relates to the preparation of N-(phosphonomethyl)glycine(“glyphosate”) from N-(phosphonomethyl)iminodiacetic acid (“PMIDA”), andmore particularly to methods for control of the conversion of PMIDA, forthe identification of reaction end points relating to PMIDA conversionand the preparation of glyphosate products having controlled PMIDAcontent.

N-(phosphonomethyl)glycine, known in the agricultural chemical art asglyphosate, is a highly effective and commercially important broadspectrum phytotoxicant useful in controlling the growth of germinatingseeds, emerging seedlings, maturing and established woody and herbaceousvegetation, and aquatic plants. Glyphosate is used as a post-emergentherbicide to control the growth of a wide variety of annual andperennial grass and broadleaf weed species in cultivated crop lands,including cotton production, and is the active ingredient in the ROUNDUPfamily of herbicides available from Monsanto Company (Saint Louis, Mo.).

Glyphosate and salts thereof are conveniently applied in aqueousherbicidal formulations, usually containing one or more surfactants, tothe foliar tissues (i.e., the leaves or other photosynthesizing organs)of the target plant. After application, the glyphosate is absorbed bythe foliar tissues and translocated throughout the plant. Glyphosatenoncompetitively blocks an important biochemical pathway that is commonto virtually all plants. More specifically, glyphosate inhibits theshikimic acid pathway that leads to the biosynthesis of aromatic aminoacids. Glyphosate inhibits the conversion of phosphoenolpyruvic acid and3-phosphoshikimic acid to 5-enolpyruvyl-3-phosphoshikimic acid byinhibiting the enzyme 5-enolpyruvyl-3-phosphoshikimic acid synthase(EPSP synthase or EPSPS) found in plants.

Various commercial processes are available for the preparation ofglyphosate. For example, glyphosate may be produced by the catalyticoxidation of PMIDA in an aqueous medium. Such reaction may be conductedin either a batch or continuous mode in the presence of a catalyst thattypically comprises particulate carbon, or a noble metal such as Pt on acarbon support. The catalyst is typically slurried in an aqueoussolution of PMIDA within a stirred tank reactor, and molecular oxygenintroduced into the reactor to serve as the oxidant. The reaction isexothermic. Temperature of the reactor is conventionally controlled bytransfer of heat from the reaction mixture to a cooling fluid in anindirect heat exchanger. The heat exchanger may comprise coils immersedin the reaction mixture within the reactor, a jacket on the exterior ofthe reactor, or an external heat exchanger through which the reactionmixture is circulated from the reactor.

Recovery of the glyphosate product typically comprises one or morecrystallization steps. The mother liquor stream or streams obtained inthe crystallization may be recycled to crystallization or reaction stepsof the process. A fraction of the mother liquor(s) is generally removedfrom the process in order to purge by-products. Crystallized glyphosatemay be dried and sold as a solid crystalline product. A substantialfraction of the glyphosate crystals are commonly neutralized with a basesuch as isopropylamine, KOH, etc. in an aqueous medium to produce aconcentrated salt solution. A concentrated formulation comprising suchglyphosate salt solution, and often also other components such as, forexample, various surfactants, is a principal product of commerce.

It is desirable to achieve substantially complete conversion of PMIDA toglyphosate during the course of the reaction. Although some unreactedPMIDA can be recovered and recycled to the reaction system, there areunavoidable losses that translate into loss of yield. The quality of theglyphosate product may also be compromised by residual PMIDA that is notremoved in the glyphosate product recovery system.

Processes have been proposed by which the reaction can be deliberatelyconducted only to partial conversion, and the resulting relatively largefraction of unreacted PMIDA separated from the reaction mixture andrecycled to the reaction system. However, processes which require therecycle of a high fraction of PMIDA involve capital intensive recoveryand recycle systems, and require relatively complicated schemes forremoval of impurities. As a consequence, it is often preferred toconduct the oxidation reaction to a high conversion, in some instancesto a substantial extinction of PMIDA.

However, it is not desirable to extend the reaction time so as tounnecessarily expose the product glyphosate to the acidic and oxidativeconditions of the aqueous reaction system. Glyphosate itself is subjectto oxidation to form the by-product aminomethylphosphonic acid (“AMPA”).Relatively extended and/or severe reaction conditions can be effectiveto drive the conversion of PMIDA to glyphosate, but can also cause aloss of glyphosate yield by further conversion of glyphosate to AMPA.Extending the reaction cycle also increases the potential for loss ofyield in the formation of N-methylglyphosate (“NMG”) by reaction ofby-product formaldehyde or formaldehyde and formic acid with glyphosate.Other impurities such as N-formylglyphosate (“NFG”),N-methylaminomethylphosponic acid (“MAMPA”) and glycine may also beformed. All these impurities and by-products may also potentiallycompromise the quality of the glyphosate product.

Consequently, there is a need in the art for methods for monitoring theconversion of PMIDA to glyphosate, and more particularly for identifyingan end point at which, or residence time over which, a target conversion(target residual PMIDA concentration) has been attained. A PMIDA contentup to about 6000 ppm by weight, basis glyphosate in the ultimateglyphosate product is typical of commercial production. In a productrecovery process comprising crystallization of glyphosate such as thatdescribed, e.g., in U.S. Application Publication No. US 2005/0059840 A1,expressly incorporated herein by reference, the PMIDA content of theglyphosate product can be maintained at less than 6000 ppm if the PMIDAcontent of the product reaction solution is not greater than about 2500ppm on a glyphosate basis.

SUMMARY OF THE INVENTION

The present invention provides multiple modifications and improvementsin a process for preparing glyphosate by the catalytic oxidation ofPMIDA. Process modifications are disclosed which provide for thepreparation of a glyphosate product having a relatively low PMIDAcontent. Certain of these modifications comprise selection and controlof reaction conditions to produce a product reaction solution ofglyphosate which has a lower PMIDA content than has been achieved innormal practice. Further in accordance with the invention, variousmethods are disclosed for monitoring the conversion of PMIDA in thecatalytic oxidation reaction, for detecting an end point of a batchoxidation reaction, for determining an appropriate residence time in acontinuous oxidation reaction, and/or for selecting and controllingreaction conditions for the production of a reaction product having arelatively low residual PMIDA content.

Briefly, therefore, in several of its aspects, the present invention isdirected to a process for the preparation of glyphosate comprisingoxidation of PMIDA or a salt thereof. The process comprises contactingPMIDA with an oxidizing agent in an aqueous reaction medium within anoxidation reaction zone in the presence of a catalyst for the oxidation,thereby effecting oxidation of PMIDA and producing a reaction solutioncomprising glyphosate or another intermediate which can be converted toglyphosate.

In various embodiments, the reaction solution is further processed toproduce a glyphosate product containing not more than about 600 ppmPMIDA or salt thereof; and in certain of these embodiments, theoxidation of PMIDA in the aqueous reaction medium is continued until theconcentration of PMIDA in the reaction medium has been reduced to aterminal concentration such that product recovery or other furtherprocessing yields a glyphosate product comprising not greater than about600 ppm by weight PMIDA, basis glyphosate.

The invention is further directed to a method of supplying a glyphosateproduct for applications in which it is desirable to maintain the PMIDAcontent of the product at consistently less than about 0.06 wt. % on aglyphosate basis. In accordance with the method, glyphosate is producedin a manufacturing facility by a process which comprises catalyticoxidation of PMIDA in an aqueous medium within an oxidation reactionzone in the presence of a catalyst for the oxidation. During designatedoperations within the facility, the process is conducted underconditions effective to consistently produce a glyphosate product havinga PMIDA content less than about 0.06 wt. %, basis glyphosate. Theproduct produced during such designated operations is segregated fromother glyphosate product produced during other operations wherein theother glyphosate product has an PMIDA content greater than about 0.06wt. %, basis glyphosate.

The invention is further directed to various methods for monitoring ordetecting the conversion of PMIDA to glyphosate or another intermediatefor glyphosate in the course of the catalytic oxidation of PMIDA in anaqueous reaction medium within an oxidation reaction zone.

One such method comprises obtaining a series of Fourier transforminfrared (“FTIR”) analyses of the PMIDA content of the aqueous reactionmedium or a sample thereof during the course of the reaction. A targetconversion of PMIDA is identified for oxidation of PMIDA to glyphosateor another intermediate for glyphosate, and/or by a target residualN-(phosphonomethyl)iminodiacetic acid content. From a plurality of FTIRanalyses, a projection is made of the batch reaction time or continuousoxidation residence time within the oxidation reaction zone at whichsaid target conversion or end point may be anticipated to be attained.

In another method for estimating PMIDA conversion or residual PMIDAcontent, a potential is applied between a working electrode and anotherelectrode immersed in the aqueous reaction medium or a sample thereof.Measurement is made of a function of the power consumed in maintaining aselect current density, or a select potential difference between theelectrodes.

According to a still further method for monitoring or detecting theconversion of PMIDA to glyphosate or another intermediate for glyphosatein the course of the catalytic oxidation of PMIDA in an aqueous reactionmedium within an oxidation reaction zone, the exothermic heat generatedin the oxidation reaction is measured; and the proportion of PMIDA thathas been converted to glyphosate or another intermediate in the reactionzone is estimated by a method comprising comparing the heat generated inthe reaction zone with the mass of PMIDA charged to the reaction zoneand the exothermic heat of reaction for the oxidation of PMIDA toglyphosate or another glyphosate intermediate.

In particular applications, the instantaneous rate of exothermic heatgenerated in the oxidation reaction zone is monitored during conversionof PMIDA to glyphosate or the another intermediate for glyphosate; andthe residual concentration of PMIDA in the aqueous reaction mediumwithin the reaction zone is estimated by a method comprising comparingthe rate of exothermic heat generation with the mass of aqueous mediumcontaining PMIDA that is charged to the reaction zone or a functionthereof. In a particular application of this method, the rate of heatgeneration is measured under conditions of non-zero order oxidation ofPMIDA to glyphosate.

Another method for monitoring or detecting the conversion of PMIDA toglyphosate or another intermediate for glyphosate in the course of thecatalytic oxidation of PMIDA in an aqueous reaction medium within anoxidation reaction zone comprises measuring the generation of carbondioxide in the reaction zone; and the proportion of PMIDA that has beenconverted to glyphosate or another intermediate in reaction zone isestimated by a method comprising comparing the carbon dioxide generatedin the reaction zone with the mass of PMIDA charged to the reaction zoneand the unit carbon dioxide generation obtained from the oxidation ofPMIDA to glyphosate or the other intermediate for glyphosate. Theestimate of conversion is made either on the basis of cumulative CO₂generation, instantaneous CO₂ generation, or a combination thereof. In aparticular application, the instantaneous rate of CO₂ generation ismeasured under conditions of non-zero order reaction.

In a PMIDA oxidation reaction wherein the oxidizing agent comprisesmolecular oxygen, the conversion of PMIDA to glyphosate and the residualPMIDA concentration can also be estimated from the cumulativeconsumption of oxygen or the instantaneous rate of oxygen consumptionduring the reaction. In certain advantageous embodiments theinstantaneous rate of oxygen consumption is measured under conditions ofnon-zero order reaction.

In various applications of the method wherein the oxygen consumption aretracked, the method is otherwise similar to or substantially the same asthat described above with respect to FTIR.

In further embodiments of the invention, the residual PMIDAconcentration or extent of conversion can be monitored by following thedissolved oxygen content or oxidation/reduction potential of the aqueousreaction medium wherein the oxidation of PMIDA takes place, or the CO₂content of the vent gas from the reaction zone. Where the oxidizingagent comprises molecular oxygen, the progress of the conversion or theresidual PMIDA concentration may be determined from the O₂ content ofthe vent gas. A high conversion and/or reaction end point is typicallyindicated by an increase in the O₂ content or a decrease in the CO₂content of the vent gas; or by an increase in the dissolved oxygencontent of the aqueous reaction medium.

In a still further embodiment of the invention, a series ofchromatographic analyses of the PMIDA content of the aqueous reactionmedium are obtained during the course of the oxidation reaction and atarget conversion of PMIDA and/or a target end point defined by a targetresidual PMIDA concentration is identified and the series ofchromatographic analyses used to project the batch reaction time orcontinuous oxidation residence time at which the target conversion orend point may be anticipated to be attained. In another embodiment formonitoring or detecting the conversion of PMIDA to glyphosate or anotherintermediate for glyphosate in the course of the catalytic oxidation ofPMIDA in an aqueous reaction medium within an oxidation reaction zone,the aminomethylphosphonic acid content of the aqueous reaction medium ismonitored and used to determine the conversion of PMIDA or identify anendpoint of the oxidation reaction.

The various methods of the invention for monitoring conversion,detecting or projecting a reaction end point, and/or detecting residualPMIDA concentration may be used in combination. Any two or more of thedisclosed methods can be combined. For example, a particular method mayprovide a cross check of one or more of the others or otherwise providea basis for refining the estimate obtained from another method orcombination of other methods.

As further disclosed herein, the methods for monitoring conversion canbe integrated into a programmed process control scheme based on analgorithm that may be based on historical operating and analytical data,and which can be updated by on-line or off-line analytical data incombination with current or historical measurement of various processparameters, including but not limited to those that form the specificbasis for the estimates of conversion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow sheet illustrating a continuous process forthe manufacture of glyphosate from PMIDA, in which the modifications ofthe present invention for production of a low PMIDA content glyphosateproduct may be implemented;

FIG. 2 is a schematic flow sheet illustrating an alternative embodimentof the process of FIG. 1 in which PMIDA that accumulates in the productrecovery area can be removed from the process in a controlled manner,more particularly in a manner that allocates PMIDA removal between asolid glyphosate acid product, a concentrated glyphosate salt solution,and a purge stream;

FIG. 3 is a schematic flow sheet illustrating an exemplary ion exchangesystem that may be used in conjunction with the process for themanufacture of glyphosate illustrated in FIG. 1 or 2;

FIG. 4 is a schematic flow sheet illustrating an evaporativecrystallization system modified to accommodate low PMIDA content in thefeed solution without excessive fouling of the heat exchange surfacesand which may be used in conjunction with the process for themanufacture of glyphosate illustrated in FIG. 1 or 2;

FIG. 5 is a family of plots of residual reactant concentration vs. timein a batch reactor (or vs. distance in a plug flow reactor), withseparate curves respectively representing zero order, first order, andsecond order kinetics, wherein the first derivative of each curve at anygiven time or location represents instantaneous reaction rate at thattime or location;

FIG. 6 is a family of plots of the log (reaction rate) vs. log (residualreactant concentration) in a batch reactor (or vs. distance in a plugflow reactor), with separate curves again representing zero order, firstorder, and second order kinetics;

FIG. 7 is a trace from a process control chart comprising a plot ofvoltage vs. time at a constant current in an electrochemical oxidationmethod for estimating the residual PMIDA content during the catalyticoxidation of PMIDA to glyphosate, and/or the end point of the catalyticoxidation reaction;

FIG. 8 comprises voltage vs. time plots similar to FIG. 7, but shows aseries of voltage vs. time curves for different select currents;

FIG. 9 is a plot of current and voltage vs. time illustrating theprinciples of a select voltage method for estimating the residual PMIDAcontent during the catalytic oxidation of PMIDA to glyphosate, and/orthe end point of the catalytic oxidation reaction;

FIG. 10 is a diagram functionally illustrating the operation of acontrol system for implementing a select voltage method for detectingthe residual PMIDA content during the catalytic oxidation of PMIDA toglyphosate, and/or the end point of the catalytic oxidation reaction;

FIG. 11 is side elevation drawing of a probe that may be used to carryelectrodes that are mounted in a process stream for measurement ofelectrochemical oxidation potential;

FIG. 12 is front elevation of the probe of FIG. 11;

FIG. 13 is a drawing in section of a pipe spool within which the probeof FIGS. 11 and 12 may be inserted;

FIG. 14 is a block diagram illustrating a scheme for controlling PMIDAconversion in a reaction system of the type illustrated in FIGS. 1 and2;

FIG. 15 is a flow sheet and instrumentation diagram for a batchoxidation reactor illustrating the controls and measurements by whichcumulative heat generation embodiments of the invention may beimplemented;

FIG. 16 is an end view of an electrode probe that is similar to FIG. 12but showing only two electrodes (i.e., a working electrode and acounterelectrode) as used in an electrochemical oxidation method fordetermining residual PMIDA or C₁ by-products;

FIG. 17 is an overlay of control chart traces of voltage vs. time atconstant current showing cyclic reversal of polarity and characteristictraces at both low conversion and high conversion in a batch oxidationof PMIDA to glyphosate;

FIG. 18 shows profiles of dissolved oxygen content, oxygen flow andvoltage required to maintain a constant current in a select currentmethod of electrochemical end point detection during a batch oxidationof PMIDA to glyphosate;

FIG. 19 schematically illustrates the stepped voltage electrochemicalmonitoring and detection method for PMIDA concentration and reaction endpoint during batch oxidation of PMIDA to glyphosate;

FIG. 20 shows the profile of O₂ and CO₂ content of the reactor vent gasvs. time during batch oxidation of PMIDA to glyphosate;

FIG. 21 shows the oxidation/reduction potential profile during batchoxidation of PMIDA to glyphosate; and

FIG. 22 is a typical plot of log (reaction rate) vs. log (residual PMIDAconcentration) in the batch oxidation of PMIDA to glyphosate, showingthat the reaction is pseudo zero order until high conversion has beenachieved, thereafter substantially first order.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred process for the manufacture of glyphosate, an aqueoussolution of N-(phosphonomethyl)iminodiacetic acid (“PMIDA”) is contactedwith an oxidizing agent in the presence of a catalyst. The catalyst maybe, for example, a particulate activated carbon as described in ChouU.S. Pat. No. 4,624,937, a noble metal on carbon catalyst as describedin Ebner et al. U.S. Pat. No. 6,417,133, or a transition metal/nitrogencomposition on carbon catalyst as described in U.S. ApplicationPublication No. US 2004/0010160 A1; International Publication No. WO2005/016519 A1; and copending and co-assigned U.S. application Ser. No.11/357,900, filed Feb. 17, 2006, entitled TRANSITION METAL-CONTAININGCATALYSTS AND CATALYST COMBINATIONS INCLUDING TRANSITIONMETAL-CONTAINING CATALYSTS AND PROCESSES FOR THEIR PREPARATION AND USEAS OXIDATION CATALYSTS (attorney's docket number MTC 6887.501), all ofwhich are expressly incorporated herein by reference.

Conventionally, the oxidation reaction is conducted in one or morestirred tank reactors wherein the catalyst is slurried in an aqueoussolution of PMIDA. The reactor(s) may be operated in either a batch orcontinuous mode. Where the reaction is conducted in a continuous mode,the aqueous reaction medium may be caused to flow through a fixed bedcomprising a catalyst for the oxidation, or through a plurality ofcontinuous stirred tank reactors (“CSTRs”) in series. The oxidizingagent is preferably molecular oxygen, though other oxidants such as, forexample, hydrogen peroxide or ozone, may also be used. Where molecularoxygen is used, the reaction is conveniently conducted at a temperaturein the range from about 70° C. to about 140° C., more typically in therange from about 80° C. to about 120° C. Where a particulate noble metalcatalyst is used, it is typically slurried in the reaction solution at aconcentration of from about 0.5% to about 5% by weight.

In a series of CSTRs, the temperature of each reactor is independentlycontrolled, but typically each reactor is operated in substantially thesame temperature range as the other(s). Preferably, the temperature iscontrolled at a level that maintains glyphosate in solution and achievessubstantial oxidation of by-product formaldehyde and formic acid,without excessive formation of either by-product iminodiacetic acid(“IDA”), which typically results from oxidation of PMIDA, or by-productaminomethylphosphonic acid (“AMPA”), which typically results fromoxidation of glyphosate. Formation of each of these by-productsgenerally tends to increase with temperature, with IDA formationoccurring principally in the first or second reactor where PMIDAconcentration is highest, and AMPA being formed principally in the lastor penultimate reactor where glyphosate concentration is relativelyhigh. Where the oxidant is molecular oxygen, it may be introducedindependently into one or more, preferably all, of the series of CSTRs.Typically, the oxygen pressure may be in the range of about 15 to about300 psig, more typically in the range of about 40 to about 150 psig.Where CSTRs are arranged for cascaded flow without intermediate transferpumps, the pressure in each successive CSTR is preferably lower than thepressure in the immediately preceding CSTR so as to assure a positivedifferential for promoting forward flow. Typically, oxygen pressure inthe first of a series of CSTRs is operated at a level approximating itspressure vessel rating, while each of the remaining reactors in theseries are operated at a pressure that is within its rating, but alsosufficiently below the pressure prevailing in the immediately precedingreactor to ensure forward flow. For example, in a system comprisingthree such reactors in series, the first reactor might be operated at apressure in the range of from about 105 to about 125 psig, the secondreactor at from about 85 to about 100 psig and the third reactor atabout 60 to about 80 psig.

A process comprising a series of CSTRs for manufacture of glyphosate isillustrated in FIG. 1. Catalytic oxidation of PMIDA is conducted in aseries of CSTRs 101 to 105 in each of which an aqueous solution of PMIDAis contacted with molecular oxygen in the presence of a particulatecatalyst slurried in the aqueous reaction medium. A reaction mixture orslurry exiting the final CSTR 105 is directed to a catalyst filter 107wherein particulate catalyst is removed for recycle to the reactionsystem. For recovery of glyphosate product, filtered reaction solutionis divided between a vacuum crystallizer 109, typically operated withoutsubstantial heat input (i.e., substantially adiabatically) and anevaporative crystallizer 111 wherein water is driven off the aqueousphase by transfer heat from a heat transfer fluid such as steam. Acrystallization slurry 113 produced in vacuum crystallizer 109 isallowed to settle, and the supernatant mother liquor 115, which containssome unreacted PMIDA, is decanted and may be recycled to the reactionsystem, typically to CSTR 101. A solid technical grade glyphosate may berecovered from the underflow slurry 117 exiting the decantation step.According to the optional process alternative illustrated in FIG. 1, theconcentrated vacuum crystallizer slurry 117 underflowing from thedecantation is divided into two fractions. One fraction 119 is mixedwith the crystal slurry exiting the evaporative crystallizer 111 anddirected to a centrifuge 121 which separates a solid crystallinetechnical grade glyphosate acid product that may be used or sold in theform of a solid wet centrifuge cake or after the centrifuge cake hasbeen dried in a conventional manner. The other vacuum crystallizerunderflow slurry fraction 123 is directed to another centrifuge 125which separates a solid crystalline product that is used to prepare aconcentrated glyphosate salt solution. For this purpose, solids 126exiting centrifuge 125 are directed to a salt makeup tank 127 where theyare neutralized with a base such as potassium hydroxide (KOH) orisopropylamine in an aqueous medium to a typical concentration of fromabout 400 to about 650 grams per liter, acid equivalent.

Mother liquor 129 from centrifuge 125 contains PMIDA in a proportionsufficient to justify recycle of at least a portion thereof to reactor101. Mother liquor 131 from centrifuge 121 is divided into a purgefraction 133, which is removed from the process, and a recycle fraction135, which is returned to evaporative crystallizer 111.

In addition to unreacted PMIDA, the oxidation reaction solutiontypically contains small proportions of other impurities that areinnocuous but generally ineffective as herbicides, and which cancompromise the crystallization step and/or reduce productivity. Thesemust ultimately be removed from the process, in part via purge 133 andin part as minor components of glyphosate products. To balance theproportion of impurities purged in fraction 133 with those removed inthe aqueous glyphosate salt concentrate product, a mother liquortransfer line 139 is provided for optional transfer of mother liquorfrom line 135 to neutralization tank 127.

FIG. 2 illustrates a modest refinement of the process of FIG. 1 whereinthe crystal slurry exiting evaporative crystallizer 111 is dividedbetween centrifuge 121 and a parallel centrifuge 137. The centrifugewet-cake from centrifuge 121 is removed from the process and may beutilized or sold as a solid technical grade glyphosate acid product, butthe wet-cake 134 from centrifuge 137 is directed to tank 127 for use inpreparing a glyphosate salt concentrate. The mother liquor draining fromboth centrifuges 121 and 137 is combined as stream 131 which is dividedbetween purge stream 133 and stream 135 which is recycled to theevaporative crystallizer.

In operation of the continuous oxidation process depicted in FIGS. 1 and2, a slurry comprising an aqueous solution comprising from about 6.5% toabout 11% by weight PMIDA is introduced continuously into CSTR 101. Theaqueous reaction medium formed in CSTR 101 may typically contain fromabout 0.5% to about 5%, more typically from about 2% to about 5% byweight of a particulate noble metal catalyst suspended therein. Forexample, the catalyst may comprise a bifunctional noble metal on carboncatalyst as described in U.S. Pat. Nos. 6,417,133, 6,603,039, 6,586,621,6,963,009, and 6,956,005, U.S. Application Publication No. US2006/0020143 A1 and International Publication No. WO 2006/031938 A2,which are expressly incorporated herein by reference. A source ofoxygen, e.g., air, or preferably oxygen enriched air or substantiallypure oxygen, is sparged into the aqueous reaction medium within reactor101 at pressure in the range from about 105 to about 125 psig andreaction is typically conducted at a temperature in the range from about90° to about 115° C. Typically, a PMIDA conversion to glyphosate in therange of about 82% to about 85% is realized in reactor 101. Reactionsolution containing slurried catalyst exiting CSTR 101 flows to secondstage CSTR 103 which is operated under substantially the sametemperature conditions as CSTR 101, but with oxygen sparged at an oxygenpressure in the range from about 85 and about 100 psig. PMIDA conversionachieved at the exit of reactor 103 is typically in the range from about90% to about 97% (i.e., conversion within the second reactor is fromabout 8% to about 15%, basis, the PMIDA charged to reactor 101).

Reaction solution with slurried catalyst exiting CSTR 103 flows to athird CSTR 105. Oxygen is sparged into reactor 105 at a pressure in therange of from about 60 to about 80 psig. Typically, the temperature ofreactor 105 is maintained in substantially the same range as reactors101 and 103. Conversion in reactor 105 is typically 3% to 5%, basis thePMIDA entering reactor 101, resulting in an overall PMIDA conversion inthe continuous reaction system from about 97% to about 99.5%.

Reactors 101 through 105 are vented under feed back pressure control. Ina preferred mode of operation, the flow rate of oxygen to each reactoris controlled to establish and maintain a target consumption of theoxygen that is introduced into the reactor in the oxidation of PMIDA andreaction by-products such as formaldehyde and formic acid. Theproportionate consumption of oxygen introduced into the reactor isreferred to herein as the oxygen utilization. In conventional operation,the pressure is preferably established at a level that provides anoxygen utilization of at least 60%, preferably at least about 80%, morepreferably at least about 90%. Consistent with the preferred oxygenutilization, the oxygen feed is divided among a series of CSTRsgenerally in proportion to the reaction rate prevailing in each of thereactors. Preferably, the reactors are sized to provide a residence timeeffective to accomplish a substantial fraction of the conversion in thefirst of a series of, e.g., three CSTRs. For example, 65% to 80% of theoxygen may be fed to the first of three reactors, 20% to 30% to thesecond, and 1% to 5% to the third. Typically, reaction in all but thelast of a series of CSTRs is pseudo zero order in PMIDA. It is believedthat this zero order reaction behavior is due to the reaction being masstransfer limited during this portion of the process. Under finishingconditions in the last reactor, the reaction is non-zero order in PMIDA,e.g., approximately first order. As discussed hereinbelow, as thecatalyst mass ages, deactivates, conversion may be maintained byincreasing the oxygen flow rates, at the same or different allocationsof oxygen among reactors (e.g., in the process of FIGS. 1 and 2 byincreasing the proportion of oxygen introduced into reactor 101 or 103),to accomplish more of the conversion in the reactors upstream of thelast reactor.

Where the oxygen utilization is relatively high, especially where it isgreater than 80% or 90%, it has been found that the PMIDA content of theeffluent from the final reactor is typically in the range of from about800 to about 2500 ppm by weight on a solution basis, i.e., from about8000 to about 25000 ppm by weight on a glyphosate basis. Whereglyphosate is recovered by crystallization from the reaction solution inthe manner described above, the PMIDA content of the glyphosateproduct(s) is generally substantially higher than in the productreaction solution exiting reactor 105. Due to recycle of mother liquorcontaining PMIDA, the aqueous crystallizer feed solutions from whichglyphosate is crystallized generally contain PMIDA in a ratio toglyphosate that is at least 25% higher, or in various steady stateoperations at least 50% higher, than the ratio of PMIDA to glyphosate inthe product reaction solution, generally substantially more than 50%higher (in a batch oxidation reaction process the PMIDA concentration inthe crystallizer feed solution may be ≥3× the concentration in theoxidation reaction solution, more typically ≥5×, even more typically≥8×). The extent of PMIDA buildup is limited by the volume of purgefraction 133. However, when a process of the type illustrated in FIGS. 1and 2 has reached substantially steady state operation, a PMIDA range offrom about 800 to 2500 ppm by weight in the final reaction solution maytypically translate into a concentration of 2000 to 6000 ppm by weighton a glyphosate basis in the final glyphosate product, provided that apurge stream is provided in which a reasonable fraction, perhaps up to10%, of the PMIDA contained in the reaction solution is purged from theprocess. Where more than one form of product is produced, e.g., whereproduct is provided in both the form of solid technical gradeparticulate glyphosate acid product and a concentrated solution of aglyphosate salt, the PMIDA content may vary between the plural products,depending in part on the direction and division of various processstreams in the product recovery scheme.

It will be understood that a variety of other schemes may be used forthe preparation of a glyphosate reaction solution by the catalyzedoxidation of a PMIDA substrate and for the recovery of glyphosateproduct(s) from a glyphosate reaction solution in the form of a solidtechnical grade glyphosate product and/or concentrated glyphosate saltsolution. For example, the filtered reaction solution may all bedirected to an evaporative crystallizer and the product recovered fromthe crystallizer slurry in a filter or centrifuge for use either asglyphosate acid or in the preparation of a concentrated glyphosate saltsolution. In such process, the mother liquor may be divided into a purgefraction and a fraction which is recycled to the evaporativecrystallizer. Alternatively, all or a portion of the mother liquor whichis not purged may be recycled to the oxidation reaction system. Variousoxidation reaction systems for the catalytic oxidation of a PMIDAsubstrate and alternative process schemes for recovering pluralglyphosate products from the oxidation reaction solution, includingschemes utilizing substantially adiabatic vacuum crystallization, areknown and described, for example, by Haupfear et al. in U.S. Pat. No.7,015,351 and U.S. Application Publication No. US 2005/0059840 A1, theentire contents of which are expressly incorporated herein by reference.

The process as described above can be routinely operated to generate aproduct reaction solution exiting the reaction system having a residualPMIDA content of not greater than about 0.25 wt. % (2500 ppm by weight),basis the reaction solution, and that the glyphosate recovered from thissolution in accordance with the process as illustrated in FIG. 2typically has a PMIDA content not greater than about 0.6 wt. % (6000 ppmby weight), glyphosate basis.

It has recently been discovered that, for some applications, it isdesirable to produce a glyphosate product having a PMIDA content notgreater than about 600 ppm by weight, glyphosate basis. It has furtherbeen determined that a concentrated aqueous solution of glyphosate saltand a solid glyphosate acid product, each having a glyphosate basisPMIDA content of <600 ppm by weight, can be consistently produced inaccordance with the process of FIGS. 1 and 2, provided that theconversion is sufficient to reduce the PMIDA content of the productreaction solution to about 45 to 60 ppm by weight plus an incrementcorresponding to whatever fraction of PMIDA may be separated in theproduct recovery system and returned to the reactor in recycle fractionssuch as mother liquor streams 115 and 129. For example, to achieve atarget concentration of 50 ppm on a glyphosate production rate basiswhere the PMIDA recycle rate is 200 ppm on the same basis (i.e., theratio of the rate at which PMIDA enters the oxidation reaction zone(s)in the recycle stream to the rate at which glyphosate flows out in theproduct reaction solution stream), a residual PMIDA concentration ofabout 250 ppm can be tolerated in the product reaction solution exitingreactor 105. In the absence of significant PMIDA recycle to the reactionsystem, as, for example, in a typical batch reaction process, the PMIDAcontent of the product reaction solution is preferably reduced to thelevel sought in the PMIDA product. However, higher levels can still betolerated where the product recovery process includes means for purginga PMIDA-enriched fraction, such as by ion exchange of PMIDA fromglyphosate as described hereinbelow.

In the commercial manufacture of glyphosate, low levels of PMIDA in theproduct reaction solution, effective to yield a glyphosate producthaving a PMIDA content not greater than about 600 ppm without excessivepurge or special separation techniques, have been occasionally andincidentally obtained on a transitory basis under certain non-steadystate operating conditions, e.g., during startup of a glyphosateprocess. However, prior to the present invention, low or otherwisecontrolled PMIDA content product has not been obtained on a deliberate,consistent or reliable basis. In sustained operation of conventionalcommercial systems for known commercial applications, generation of suchextra low PMIDA content glyphosate would not have been justified,because of the penalties in yield, productivity and/or quality (withrespect to by-product impurities such as AMPA and NMG) incurred underthe conditions which might have been effective for such sustainedoperation.

In accordance with the invention, various process stratagems have beendiscovered for consistently and permanently controlling the PMIDAcontent of the product reaction solution and particularly at a levelbelow about 60 ppm, or below the sum of about 60 ppm and an increment,if any, corresponding to recycle of unreacted PMIDA from other processoperations, for example, in recycle to the oxidation reaction zone fromthe glyphosate product recovery operation in the continuous processillustrated in FIGS. 1 and 2 or from other further processing steps suchas conversion of PMIDA to the N-oxide in accordance with the processesof U.S. Pat. Nos. 5,043,475, 5,077,431 and/or 5,095,140. In addition orin the alternative to such modifications to the reaction system,modifications to the glyphosate recovery process have been devised torecover a low PMIDA content glyphosate product, e.g., <600 ppm, ineither one of a plurality of products, or in all the glyphosate productsof the process.

Other schemes have been developed for generating a reaction solutionhaving an unreacted PMIDA content <250 ppm on a campaign basis, and/ormodifying the glyphosate recovery process on a campaign basis to producea glyphosate product having a PMIDA content <600 ppm.

Modifications in PMIDA Oxidation Reaction Conditions and Systems

In accordance with the present invention, it has been discovered thatoxygen flow to the reactor(s) may be optionally adjusted in a mannerthat reduces the concentration of PMIDA in the final reaction solution,resulting in a generally proportionate decrease in the PMIDA content ofthe glyphosate product or products. Generally, it has been found thatincreasing oxygen flow in one or more of the reactors enhances theconversion of PMIDA to glyphosate. The exact relationship of oxygen flowto PMIDA conversion varies significantly with the other conditions ofthe process, with the nature of the catalyst, with catalyst age andconcentration, with batch vs. continuous operation, with productthroughput, and with the peculiarities of the configuration of aspecific reactor, its oxygen feed point, agitation system and gas flowpatterns. However, those skilled in the art can readily adjust theoxygen flow rate for a specific reactor or series of reactors to obtaina desired response in increased conversion of PMIDA. By way of example,where a continuous reaction system of the type illustrated in FIG. 1 isoperating at a residual PMIDA level of 800 to 1500 ppm in the reactionsolution exiting CSTR 105, the PMIDA content of the product reactionsolution may be reduced to from about 150 to about 250 ppm by aproportionate increase in the sum of the oxygen flow rates to reactors101 to 105 of roughly from about 0.1 to about 2% relative to the sum offlow rates that yields a PMIDA content of 800 ppm under otherwiseidentical process conditions, or by adjusting the reaction temperatureor the agitation intensity in the reaction zone. Alternatively, suchreduction in PMIDA content of the product reaction solution may beachieved by increasing the flow rate of oxygen to the last of the seriesof reactors, reactor 105, by at least about 5%, typically from about 10%to about 30% relative to the flow rate which yields a PMIDA content of800 ppm under otherwise identical reaction conditions.

Over an extended period of operations, the catalyst may deactivate tothe extent that desired conversion can no longer be achieved byadjustment of oxygen flow to the last of a series of CSTRs. However, upto a limit defined by useful catalyst life (or at augmentation orpartial replacement with fresh catalyst), the desired conversion canstill be maintained by progressively increasing the oxygen flow to theearlier reactors, e.g., reactors 101 and 103 in FIGS. 1 and 2.Preferably, the oxygen flow rate is increased sufficiently to actuallyincrease the conversion in the reaction solution exiting the penultimatereactor, so that the duty imposed on the last reactor is reduced. Thus,the desired ultimate conversion is obtained even though the productivityof the last reactor per se has declined. Conversion can also beincreased by increasing residence time in the reactors. As those skilledin the art will appreciate, an infinite number of combinations of flowrates to the respective reactors may be available to achieve the desiredlevel of PMIDA in the product reaction solution.

In a batch reaction system, the PMIDA content of the reaction solutionmay optionally be reduced by extending the cycle during which a sourceof oxygen is sparged into the aqueous reaction medium. For a givenoperation, a conventional oxygen flow cycle may be identified by anyconvenient conventional means, as, for example, by periodic analysis ofsamples from the reactor. Where performance as a function of time isreasonably consistent, timing of the batch may be sufficient andsampling may not be necessary. In any case, it has been discovered that,by extending the oxygen sparging cycle by from about 2 to about 15minutes, more typically from about 5 to about 10 minutes, PMIDAconversion can be increased to reduce residual PMIDA content from arange from about 275 to about 350 ppm to a range from about 50 to about100 ppm or even lower.

The achievement of a low PMIDA content in the filtered aqueous reactionproduct stream by increased oxygen flow or extended batch cycletypically involves a modest penalty in glyphosate yield and an increasein the concentration of certain impurities, prominentlyaminomethylphosphonic acid (“AMPA”). Where a noble metal on carboncatalyst is used for the reaction, these schemes may also typicallyresult in an increased rate of deactivation of catalyst, resulting inincreased catalyst consumption. However, the reduced PMIDA contentgenerally affords a benefit in those applications in which it isdesirable to utilize a glyphosate product having a low PMIDA content,such as in the preparation of herbicidal glyphosate compositions for thecontrol of weeds in genetically-modified cotton crops, that outweighsthe adverse effects on yield and the minor increase in impurities.

In accordance with the invention, several additional modifications tothe reaction system have been identified that can be used in lieu of, orin combination with increased oxygen flow as described above.

Alternatively, or in addition to increasing oxygen flow to thereactor(s), enhanced conversion of PMIDA can be achieved by operation atrelatively high reaction temperature within the aforesaid range of fromabout 70° to 140° C., and/or by modification of the catalyst system.

Conversion of PMIDA is promoted by operation at elevated temperature,e.g., in the range of about 110° C. or above, typically from about 110°to about 125° C. Because higher temperature leads to increasedby-product formation, such as by oxidation of glyphosate to AMPA, thetemperature is preferably not increased to more than the extent that maybe necessary, either alone or in combination with other modificationssuch as oxygen flow rate, to achieve the target level of PMIDA. Asignificant effect on conversion can be achieved by operation in therange of from about 115° to about 125° C., or perhaps optimally in therange of from about 118° to about 125° C.

The catalyst system may be modified by an increased charge of noblemetal on carbon catalyst, by adding activated carbon to the catalystsystem and/or by altering the selection of promoter for the noble metalon carbon catalyst. It has also been found that the catalyst activitymay be enhanced by selection of calcination conditions, control of thecalcination atmosphere, and other conditions prevailing during thepreparation of a noble metal on carbon catalyst, as described, forexample, in International Publication No. WO 2006/031938 A2. If a freshcatalyst charge is increased beyond a threshold level, e.g., above aconcentration in the range of from about 1.5% to about 2% by weight, theeffect may be to increase the oxidation of PMIDA to IDA rather thanglyphosate. However, while PMIDA may oxidize to IDA resulting in anoverall selectivity loss, the net effect is still to reduce the PMIDAcontent of the final glyphosate product. Moreover, when a catalyst masshas been used through a substantial number of recycles, activity of thecatalyst mass may usefully be increased by purging some fraction of thespent catalyst and adding fresh catalyst in its place. When this methodis followed, PMIDA conversion may be significantly enhanced withoutsignificant formation of IDA, i.e., selectivity to glyphosate may besubstantially preserved.

An activated carbon catalyst such as the catalyst that is described byChou in U.S. Pat. No. 4,624,937, is highly effective for oxidation ofPMIDA to glyphosate, even if not as effective for oxidation ofby-product C₁ species such as formaldehyde and formic acid. The carboncatalyst is also relatively inexpensive compared to the noble metal oncarbon catalyst, though it is typically consumed at a substantiallyhigher rate. Thus, a fairly liberal addition of carbon catalyst toeither a batch reactor, or to the last of a series of cascaded CSTRs,(e.g., in a proportion of at least about 1.5% by weight, typically fromabout 2.5% to about 3.5% by weight, basis, the noble metal on carboncatalyst charge) can materially reduce the residual PMIDA content in thefinal reaction solution.

Certain transition metals such as Bi and Te are effective as promotersto improve the effectiveness of a noble metal on carbon catalyst foroxidation of by-product C₁ species such as formaldehyde and formic acid.However, data indicate that the oxidation of PMIDA may be marginallyretarded by such promoters, perhaps by directing oxygen to contact andreact with C₁ species in preference to PMIDA. When used either alone orin combination with activated carbon for preparation of low PMIDAcontent glyphosate, a noble metal catalyst can either have no promoter,or have a promoter whose identity and loading is selected to minimizeany negative effect on the kinetics of the PMIDA oxidation. In thisconnection, a particular reactor, such as the final reactor in a seriesof CSTRs, can be dedicated to substantial extinction of PMIDA, and theuse of a catalyst which has no promoter, or in which the promoter isselected to be favorable to PMIDA oxidation, can be limited to thededicated reactor.

Because further thermal effects are minimal once a relatively highconversion has been achieved, a finishing reactor, such as the finalreactor in a series of continuous reactors, can readily be operated as aflow reactor, e.g., with a fixed catalyst bed, rather than a back-mixedreactor, so as to enhance the driving force for extinction of PMIDA.Moreover, such finishing reactor can be added, for example, as reactorn+1 after a series of n CSTRs, for example as the fourth reactorfollowing reactor 105 of FIG. 1. Optionally, the catalyst loaded in suchreactor can predominantly or exclusively comprise activated carbon.

In order to minimize residual PMIDA in the product reaction solutionexiting the final stage of a cascaded continuous stirred tank reactionsystem, it is helpful to minimize short circuiting of aqueous mediumfrom the reactor inlet to the reactor exit. Thus, in accordance withprinciples known to the art, the feed point, exit point, baffle array,agitation pattern and agitation intensity may be selected to minimizethe extent of short circuiting. Where a CSTR is provided with anexternal heat exchanger through which the reaction mixture is circulatedfor removal of the heat of reaction, the reaction mixture mayconveniently be withdrawn from the reactor at a forward flow port in thecirculating line. Advantageously, the inlet for reaction medium can bepositioned in the same circulating line downstream of the exit port by adistance sufficient to avoid any short circuiting due to axialbackmixing. For example, the exit port can be placed in the circulatingline upstream of the heat exchanger and the inlet port can be locatedimmediately downstream of the heat exchanger.

In accordance with the invention, further process modifications outsidethe principal PMIDA oxidation system, may be used to reduce the PMIDAcontent of the finished glyphosate product(s). Such additionalmodifications, as described hereinbelow, may be used together with or inlieu of any combination of the modifications to the reaction system thatare described above.

PMIDA Purge

For example, in the process of FIG. 1, the volume of purge streamfraction 133 can be increased relative to evaporative crystallizermother liquor recycle fraction 135, thus reducing the steady stateinventory of PMIDA in the glyphosate product recovery area of theprocess. The extent of purge required to obtain a given specificationfor a given form of glyphosate product varies depending on the PMIDAcontent of the filtered reaction product stream and the exact materialbalance of the overall process, and especially the material balance ofthe glyphosate recovery area. The effect of increased purge may beaugmented by a more extended wash of the separated glyphosate solidsthat are obtained as a centrifuge cake in centrifuges 121 and 125, or infilters or centrifuges that may be used in alternative schemes forproduct recovery. Increased wash volume is ordinarily integrated withthe purging scheme because either the wash liquor itself must be purged;or, if the wash liquor is combined with one or more of the recyclemother liquor streams, it marginally increases the amount of PMIDA whichmust be purged from the process. In either case, the net purge volume isgenerally increased by an increment corresponding to the volume of thewash liquor. An increase of wash volume might be achieved independentlyof the purge fraction where the quality of the wash solution permits itsuse in preparing the aqueous solution of PMIDA which is introduced intothe reaction system.

Ion Exchange

In a further alternative embodiment of the invention, PMIDA may beremoved from one or more process streams by ion exchange. A variety ofoptions may be followed in providing for removal of PMIDA by ionexchange. For example, an ion exchange column could be used to removePMIDA from mother liquor as it is recycled from the evaporativecrystallizer centrifuge 121 (and/or 137) before separation of purgefraction 133, or in recycle mother liquor fraction 135 after separationof the purge fraction, or in stream 129 from centrifuge 125.Alternatively, or additionally, an ion exchanger could be positioned inthe filtered reaction solution stream ahead of the point where it isdivided between the vacuum crystallizer 109 and the evaporativecrystallizer 111 in FIG. 1.

In an ion exchange system, the PMIDA-bearing stream is contacted with ananion exchange resin, preferably an anion exchange resin which has agreater affinity for the more strongly acidic PMIDA anion than for therelatively more weakly acidic glyphosate anion and for many of the othercompounds in this stream. Because the process stream from which PMIDA isto be removed typically has a high ratio of glyphosate to PMIDA, theresin's affinity for PMIDA should be significantly greater than itsaffinity for glyphosate. Efficient separation of PMIDA is enhanced wherethe affinity of the resin for PMIDA is at least two times, three times,four times, five times, 10 times, 20 times or as much as 100 times itsaffinity for glyphosate. Weakly basic exchange resins are preferred.Functional sites of conventional weak base anion exchange resinstypically comprise secondary amine or tertiary amines. Available anionexchange resins typically comprise, e.g., a styrene butadiene polymerhaving a secondary or tertiary amine site which may be protonated inacidic solution to function as an anion exchanger. Suitable commerciallyavailable resins include, for example: AMBERLYST A21, AMBERLITE IRA-35,AMBERLITE IRA-67, AMBERLITE IRA-94 (all from Rohm & Haas, Philadelphia,Pa.), DOWEX 50 X 8-400 (Dow Chemical Company, Midland, Mich.), LEWATITMP-62, IONAC™ 305, IONAC™ 365 and IONAC™ 380 (Sybron Chemicals,Birmingham, N.J.), and DUOLITE™ a-392 (Diamond Shamrock Corp., Dallas,Tex.).

A complication in removal of PMIDA by ion exchange can arise from thepresence of a substantial fraction of chlorides in the filtered reactionsolution, which tend to be concentrated somewhat in the solutionultimately subjected to ion exchange such as evaporative crystallizermother liquor 131. When an acidic solution such as mother liquor recyclesolution 131 is passed over an anion exchange resin, chloride ions areretained at the protonated amine sites preferentially to PMIDA. Wherethis is the case, two columns may typically be provided in series, withthe first column dedicated to removal of chloride ions, with either astrong or weak base anion exchange resin, and the effluent from thefirst column passed through a second column comprising a weak baseexchange resin wherein PMIDA anions are removed. Each column may beeluted and the anion exchange resin regenerated by passage of a causticsolution, typically sodium hydroxide (NaOH), through the column.

The solution from which PMIDA and/or chlorides are to be removed ispassed through the column in which the desired exchange occurs untilbreakthrough of the ion to be removed is observed in the effluent fromthe column. Breakthrough may occur when the entire column has reached anequilibrium level of chloride ion or PMIDA as the case may be. Assaturation is approached, the capacity of the column for the targetanion may be reduced to some extent by the presence of the anions ofcomponents that are of comparable acidity as the target anion, e.g.,phosphate and N-formylglyphosate (“NFG”). Breakthrough may be determinedby any conventional means of detection, including, for example,conductivity, absorbance of light (254 nm), pH and the like. In apreferred method, PMIDA breakthrough is detected by monitoringconductivity of the column eluate. For example, as described in greaterdetail below, a potential may be applied between a working electrode andanother electrode immersed in the column eluate or a sample thereof, andmeasurement made of a function of the power consumed in maintaining aselect current density, or a select potential difference between theelectrodes. Alternatively, the end point of an ion exchange cycle can bepracticed by volumetric control of the quantity of aqueous solutionpassed through the column (i.e., the cumulative quantity of motherliquor or other PMIDA-containing stream passed through the anionexchange bed relative to the volume of the bed, typically expressed in“bed volumes.”).

After an ion exchange cycle is complete, the column can be eluted toremove the anion that has been collected therein.

A column in which chlorides have been collected may be eluted with acaustic solution, e.g., NaOH, to regenerate free amine sites and producean eluate salt solution that may typically be discarded. Interstitialcaustic is removed by washing the column with water. Unless interstitialcaustic is removed, it is recycled to the crystallizer with adverseimpact on the crystallization.

A column in which PMIDA has been collected may first be washed withwater to displace process liquid from the column. Thereafter the columnmay be eluted with a strong acid such as HCl to remove PMIDA forrecovery; and then regenerated, typically with a caustic solution suchas NaOH, and then washed with water to remove interstitial caustic.Eluate comprising PMIDA can be recycled to the reaction system forfurther conversion of the PMIDA to glyphosate. Illustrative examples ofacids which can be used for elution of PMIDA from an ion exchange columninclude strong mineral acids such as hydrochloric acid or sulfuric acid.In various embodiments, the ion exchange resin may be contacted with awash solution, or multiple wash solutions during a series of wash stepssubsequent to elution. Suitable wash solutions include, for example,water, a buffer solution, a strong base such as KOH, NaOH, or NH₄OH or aweaker base such as Na₂CO₃.

During elution of a column loaded with PMIDA, the column effluent ismonitored for the conjugate base of the strong acid, e.g., chloride ionwhen Cl⁻ is detected in the effluent. Upon appearance of chlorides,recycle of eluate to the PMIDA oxidation step is terminated, and thecolumn is washed with water, then caustic and then again water to returnit to the free amine state. If desired, buffers and/or solvents may beused in washing of the column after elution, but this is not ordinarilynecessary or useful.

Ion exchange can be conducted at ambient or elevated temperature. Moreparticularly, the mother liquor from the evaporative crystallizercentrifuge 121 (and/or 137) may be treated by ion exchange resin withoutheating or cooling prior to introduction into the ion exchange column.Typically, this stream has a temperature in the range of from about 45°to about 85° C., more typically from about 55° to about 75° C. Columndimensions and flow rates through the column are governed by standardcolumn design principles and can be readily determined by one skilled inthe art.

If desired, a third column can be provided downstream of the PMIDAcolumn for recovery of glyphosate by ion exchange. See, for example, theprocess as described in U.S. Pat. No. 5,087,740, which is expresslyincorporated by reference herein.

In various embodiments, a still further ion exchange column may beprovided for recovery of platinum or other noble metal that may havebeen leached from the catalyst used in the oxidation of PMIDA. A processfor recovery of such noble metal by ion exchange is described incopending and co-assigned U.S. application Ser. No. 11/273,410, filedNov. 14, 2005, entitled RECOVERY OF NOBLE METALS FROM AQUEOUS PROCESSSTREAMS (attorney's docket number MTC 6909.1), which is also expresslyincorporated herein by reference. Preferably, ion exchange for recoveryof noble metal is conducted upstream of the ion exchanger used forseparation of PMIDA or removal of chlorides.

In a continuous process such as that illustrated in FIG. 1, a pair ofion exchange columns can be provided in parallel for each ion exchangeoperation that is conducted as part of the process. In this manner, onecolumn can be used for removal of target anion while the other is beingeluted and regenerated.

Although ion exchange has been described above with reference to ionexchange columns, the resin may alternatively be added directly withagitation as a solid phase reagent to the stream from which the PMIDA(or other target anion) is to be removed. Ion exchange operations havebeen described above with reference to the continuous processes depictedin FIGS. 1 and 2. Removal of excess PMIDA by ion exchange is also usefulin a simplified glyphosate product recovery scheme in which all productreaction solution is directed to a single glyphosate recovery stage suchas a single evaporative crystallizer. A single crystallizer typicallymay be used where the oxidation reaction is conducted in a batch mode.In such a process, glyphosate crystals are separated from thecrystallization slurry by filtration or centrifugation, and the motherliquor typically recycled to the crystallizer. In extended operations, afraction of mother liquor is purged to remove impurities. Ion exchangefor removal of PMIDA from the mother liquor allows reduction of thepurge fraction necessary to provide a given PMIDA specification in theglyphosate product. Also, the PMIDA which removed can be recovered byelution as described above and recycled to the oxidation reactor.

FIG. 3 illustrates an exemplary ion exchange system, located, forexample, in stream 131 of FIG. 1 or 2, upstream of the purge 133. Asillustrated, the system comprises three columns in series, a platinum(or other noble metal) recovery column 201, a chloride removal column203 and a PMIDA removal column 205. Column 201 comprises an adsorptionzone which may comprise activated carbon, or more typically a weak baseanion exchange resin, strong base anion exchange resin, strong acidcation exchange resin weak acid cation exchange resin, chelating resin,or in some instances, mixtures thereof. Specific resins useful in therecovery of solubilized platinum are described in U.S. Ser. No.11/273,410 (attorney's docket number MTC 6909.1), expressly incorporatedherein by reference. Preferably, a chelating resin is used. Column 203comprises an anion exchange zone containing a resin of the typedescribed hereinabove for removal of chlorides, and column 205 comprisesan anion exchange zone containing a resin of the type described for theremoval of PMIDA.

Although only a single column is depicted for each recovery or removaloperation in FIG. 3, typically at least a pair of columns is provided inparallel at each stage to allow one column to be eluted, regenerated,and washed while the other is in operation for removal of Pt, Cl⁻ orPMIDA, respectively. Operating conditions for column 201 are describedin U.S. Ser. No. 11/273,410. As further described in the above-mentionedapplication, breakthrough of noble metal from column 201 may be detectedby ICP-MS, ICP-OES, or AA. A simple conductivity device is effective fordetermining breakthrough of chlorides from column 203 or 205.

While FIG. 3 depicts separate columns (or column pairs) in series forchloride removal and PMIDA removal, respectively, the two columnsfunction as a single adsorption system so far as adsorption phenomenaare concerned, at least in the case where the ion exchange properties ofthe resins used in columns 203 and 205 are substantially the same. Inany case, all adsorbable components of the solution are initiallyadsorbed on column 203 but PMIDA is progressively displaced by Cl— asthe column becomes loaded. PMIDA desorbed from or passing through column203 is adsorbed on the anion exchange resin in column 205. When column203 (or a corresponding adsorption zone within a single column) becomesloaded with chloride, the latter ions eventually break through in theeffluent from column 203 (or corresponding zone) and begin displacingthe PMIDA from column 205 (or a corresponding downstream adsorption zoneof a single column). Separating the adsorption bed into two columnsfacilitates monitoring the chloride wave and scheduling regeneration ofanion exchange resin for sustained operations. Breakthrough from column205 may result from either saturation of the resin therein with PMIDA ordisplacement of PMIDA by chloride. In either case, breakthrough mayoccur before maximum PMIDA loading is realized, with the PMIDA contentof the effluent progressively increasing as column saturation isapproached, rising to the level in the inlet mother liquor stream whensaturation is reached. Where chloride displaces PMIDA, the PMIDA loadingreaches a maximum and then begins to decline as it is displaced bychloride. In the system depicted in FIG. 3, this condition can beavoided if column 203 is regenerated as soon as chloride breakthrough isobserved. In either case, process operators can identify an optimumbalance between PMIDA removal efficiency and column loading.

Regardless of whether the chloride and PMIDA ions are removed inphysically separate adsorption beds in series or in a single adsorptionbed, the adsorption system may be considered to comprise two distinctadsorption zones, one in which chlorides are being adsorbed and anotherin which PMIDA is being adsorbed. However, the size and location ofthese adsorption zones are not static. The boundary between the zonesmoves as the chloride wave advances in displacing PMIDA from the resin.

Shown at 207 is a device effective to sense breakthrough of PMIDA fromcolumn 205. The device comprises a pair of electrodes immersed in thestream exiting the column or a sample thereof, and is controlled tomaintain a select current density or impose a select voltage or scheduleof voltages between the electrodes. Where the device is controlled tomaintain a select current density, breakthrough of PMIDA is reflected ina drop, typically a relatively sharp drop in the voltage required tomaintain the select current density. Where a select voltage, orprogrammed series of voltages is imposed, breakthrough of PMIDA isindicated by a significant increase in current at a voltage that issufficient for electrolytic oxidation of C₁s and PMIDA but not residualglyphosate. Detailed descriptions of devices which function on thesebases are set forth in greater detail below.

Whenever any of columns 201, 203 or 205 reaches a breakthroughcondition, introduction of mother liquor is terminated and the adsorbedcomponent recovered. In the case of column 205, PMIDA may be eluted witha strong acid such as HCl. Both columns 203 and 205 may be regeneratedusing a caustic eluant, followed by a water wash, as described above.The aqueous NaCl eluate may be discarded. In the case of column 201, thenoble metal component may optionally be eluted with an eluant, e.g., anacidic eluant where the noble metal species is present in the form ofcation, or a caustic eluant where the noble metal is present in ananion. However, in the case of column 201, more quantitative recoverycan generally be achieved by removing the loaded resin from the column,incinerating the resin, and recovering noble metal from the ash.

Recovery of noble metal in column 201 is typically in the range betweenabout 60% and about 85%, or even higher. Thus, in monitoring operationof this column “breakthrough” is a relative term, and the breakthroughdetection device is calibrated to detect an increase in signal above asteady state level. In any event, a portion of the noble metal istypically lost in purge stream 133 or in the product glyphosate saltconcentrate. Where PMIDA is removed by ion exchange via column 205, ithas been found that a portion of the noble metal passing through columns201 and 203 is adsorbed on the resin contained in column 205. If thiscolumn is regenerated or washed with aqueous ammonia, the platinum isdesorbed, and ultimately lost either in the purge stream or byincorporation into the aqueous glyphosate salt product. However, it hasbeen discovered that if the column is regenerated with a strong basesuch as an alkali metal hydroxide, e.g., NaOH or KOH, and washed withstrong base or water, platinum species are typically not desorbed, butremain on the column, thus allowing ultimate recovery of this fractionof the platinum by removal and incineration of the resin.

Disposition of the eluates from columns 203 and 205, respectively, is asdescribed above. The acidic eluate comprising PMIDA is typicallyrecycled to the reaction system. As regeneration proceeds, the chloridecontent typically declines in the caustic regeneration solution exitingthe column. Advantageously, a portion of the caustic regenerationsolution, particularly that exiting the column toward the end of theregeneration cycle, may be preserved and used in a subsequentregeneration cycle in the same or a parallel PMIDA removal column.Although an anion exchange resin which has a substantially higheraffinity for PMIDA than for glyphosate is preferably selected for column205, some glyphosate is typically removed along with PMIDA from themother liquor or other solution that is processed in the column. Theincidence of glyphosate removal may be relatively significant when thecolumn contains fresh or freshly regenerated resin. As PMIDA accumulatesin the column, the glyphosate fraction moves down (or in any eventtoward the column exit) in a manner similar to the operation of achromatographic column. In an alternative embodiment of the process, theeffluent from column 205 may be monitored not only for PMIDA but alsofor glyphosate. As the column becomes loaded with PMIDA, glyphosatebreaks through first. When the column is eluted, a glyphosate fractioncomes off first and may be segregated for recycle, e.g., to theevaporative crystallizer. Prior to elution, the column is washed forremoval of residual glyphosate caught in the interstitial spaces betweenthe resin beads. The glyphosate content of the wash solution may also besufficient to justify recycle to the evaporative crystallizer.

Where the operation of column 205 is monitored by use of device 207, thethreshold voltage at which a significant current density is realized mayfirst be observed to decline to a value reflective of the oxidation ofglyphosate. Such threshold voltage substantially prevails until PMIDAbreakthrough approaches. During elution, a similar voltage response orrequirement should be observed during elution of the glyphosate fractionwhich may be directed, e.g., to a feed tank for the evaporativecrystallizer. When the voltage required to sustain a target currentdensity declines to a value reflective of the oxidation of PMIDA, theeluate may be redirected for recycle to the reactor, or alternatively tothe purge.

According to a further alternative for recovery of glyphosate, a columnloaded with both glyphosate and PMIDA may be initially eluted with arelatively weak base such as isopropylamine (“IPA”) to remove therelatively weakly sorbed glyphosate in the form of the salt. Optionallyand preferably, neat liquid IPA can be used for the elution, whichproduces an eluate consisting of a relatively concentrated solution ofthe IPA salt of glyphosate. This eluate may be directed toneutralization and mixing tank 127 and used directly in producingaqueous IPA glyphosate concentrates.

In accordance with a further process alternative, as mentioned above,another column comprising an ion exchange zone comprising a resineffective for sorption of glyphosate, typically a further ion exchangecolumn, can be provided downstream of column 205. This column is notshown in FIG. 3 but may be positioned to receive the process stream thathas been passed in series through columns 203 and 205, or in seriesthrough columns 201, 203 and 205.

Polishing Reactor in Product Recovery Process

According to a further alternative, PMIDA can be removed from productrecovery process streams by catalytic oxidation to glyphosate. Inaddition to or in lieu of a finishing reactor as described above in theprincipal reaction train, polishing reactor(s) can be positioned in oneor more process streams within a product recovery system of the typeillustrated in FIG. 1. For example such a reactor could be positioned inthe feed stream to evaporative crystallizer 111 (as a pre-recoverypolishing reactor), in mother liquor stream 131 exiting evaporativecrystallizer centrifuge 113, or elsewhere in the process.

Such a further finishing or polishing reactor can optionally be operatedwith a carbon only catalyst. Moreover, since only marginal oxidation isinvolved, thermal effects are minimal, making it at least potentiallyadvantageous to operate the reactor as a flow reactor with a fixed bedof catalyst, thus enhancing the driving force for substantial extinctionof PMIDA. Where the reactor is placed in stream 131, ahead of the purgestream, the effect on overall yield of the marginal oxidation ofglyphosate to AMPA is minimal. Oxidation reaction systems forpreparation of glyphosate reaction solutions by catalytic oxidation of aPMIDA substrate including finishing or pre-recovery polishing reactorsare described by Haupfear et al. in U.S. Pat. No. 7,015,351, the entirecontents of which is incorporated herein by reference.

According to a still further alternative, the process may typicallyinclude a feed tank for the catalyst filter 107, and a further marginalreduction in the residual PMIDA content may be realized by sparging anoxygen-containing gas, e.g., substantially 100% O₂ into the contents ofthe filter feed tank, or into the slurry of catalyst in product reactionsolution in a line entering the feed tank. In order to prevent settlingof catalyst in the filter feed tank, its contents are typicallyagitated, which may aid in oxygen distribution, mass transfer andconsequent oxidation of residual PMIDA. Oxygen sparging may contributeto agitation.

Crystallizer Operations

Process options effective to produce a product of relatively low PMIDAcontent have implications for the operation of evaporative crystallizer111. PMIDA has been found to function as a solubilizer for glyphosate.Thus, where the reaction system is operated under such conditions as toyield a filtered product reaction solution of relatively low PMIDAcontent, and/or where the filtered reaction solution is passed through afinishing reactor for further conversion of PMIDA to glyphosate, and/orwhere PMIDA is removed from recycle mother liquor by ion exchange,solubility of glyphosate in the recycle mother liquor can be lowered. Ata given system pressure, a lower PMIDA/glyphosate ratio causescrystallization to commence at relatively lower temperature, which canresult in fouling of process side heat exchanger surfaces in orassociated with the evaporative crystallizer.

FIG. 4 illustrates an evaporative crystallization system modified toaccommodate low PMIDA content in the feed solution without excessivefouling of the heat exchange surfaces. In the system of FIG. 4,crystallizer 109 comprises a vapor liquid separator 301, an externalheat exchanger 303, and an axial or centrifugal circulation pump 305 andline 307 for circulation of the crystallization slurry between the vaporliquid separator through the heat exchanger. A mist eliminator 309 inthe upper portion of the vapor liquid separator helps to collectentrained liquid and return it to the liquid phase within the separatorbody. Crystallization slurry is drawn off through port 311 in thecirculation line for delivery to centrifuge 121 and optionallycentrifuge 137. Fouling of heat exchanger 303 is potentiallyattributable to accumulation of glyphosate on the process side tubesurfaces, but may also be attributable to plugging of the heat exchangertubes with large chunks of crystalline material which may calve off thewalls of separator 301.

It may further be noted that the commencement of crystallization atlower temperature results in an enhanced crystallization yield. Whilethis effect may be advantageous from the standpoint of initialcrystallizer productivity, and marginally beneficial with regard toyield on raw materials, the higher solids content of the circulatingslurry is believed to have an adverse effect on heat transfer. Increasedsolids content increases the effective viscosity of the circulatingslurry, thereby increasing pressure drop through the heat exchanger. Ata given limiting pump head, this results in a decreased flow rate,decreased velocity along the process side of the tube wall, andconsequently decreased heat transfer coefficients. Thus, even withoutany fouling or plugging of tubes, heat transfer rates and productivitycan be compromised by the higher solids content obtained as thecrystallization temperature drops with PMIDA content.

In any event, injection of water into the circulating pump suctionimposes a sensible heat load that tends to reduce the rate ofprecipitation in the tubes. Although water injection does not reduce thesteady state composition of the liquid phase in the vapor/liquidseparator, it marginally reduces the degree of supersaturation in theliquid phase entering the heat exchanger, and may thus marginally reducethe tendency of the tubes to foul by further encrustation withglyphosate. Perhaps more significantly, it reduces the solids content ofthe slurry passing through the heat exchanger, thus reducing theviscosity, and contributing to increased process side velocity and heattransfer coefficients.

Injection of water above the mist eliminator is useful in minimizingpressure drop through the mist eliminator and controlling the extent ofcrystallization on the walls of the separator. Increasing the slurrycirculation rate via pump 303 serves to reduce the temperature rise inthe heat exchanger and enhance the scouring action of the circulatingslurry, further contributing to control of fouling.

Aside from the complications which it can create in the operation of theevaporative crystallizer, ion exchange also functions to reduce thechloride and phosphate content of the mother liquor circulating in theevaporative crystallization system. Whether as a result of lowerchloride and phosphate content or otherwise, it has been found thatenhanced crystal growth is achieved in the evaporative crystallizer inoperations wherein PMIDA, and necessarily also chloride and phosphate,is removed by ion exchange. The larger crystals thus produced havesuperior dewatering properties as compared to the crystals obtained inan evaporative crystallization system wherein a mother liquor ofrelatively high PMIDA, Cl—, and/or phosphate concentration circulatesbetween the evaporative crystallizer and centrifuge 121 or 121 and 137.Production of relatively larger crystals is advantageous in removal ofresidual impurities, including PMIDA, by separation of solids frommother liquor in the centrifuge(s) and washing of the centrifuge cake.It has further been observed that, where the crystallizer is operated toconsistently generate relatively large glyphosate particles, the foulingeffect of reduced PMIDA content is at least partially offset. Heatexchange surfaces are generally less prone to fouling in an operationwherein heat is transferred to a slurry comprising relatively largeparticles than in an operation where relatively fine crystals areproduced.

Designated Operations for Low PMIDA Glyphosate

Certain of the process modifications as described above are effectivefor producing low PMIDA content glyphosate, but involve risks withregard to yield, productivity, product quality and catalyst deactivationas affected by the presence of by-product impurities such as AMPA, NMG,etc. Thus, for example, while modifications comprising increased oxygenflow or extended batch cycles or higher temperatures are effective forlowering the PMIDA content, they involve risk of overreaction, resultingin the oxidation of glyphosate to AMPA, or the formation of by-productssuch as NMG. Modifications to the material balance of the glyphosaterecovery system, such as those discussed above in connection with FIG.2, can result in yield losses due to increased purge of unreacted PMIDAfrom the process, and the direct loss of glyphosate yield that isassociated with the glyphosate content of the purge stream.

Further in accordance with the invention, operation of the process canoptionally be managed to consistently and reliably meet demand for lowPMIDA glyphosate product, while minimizing the net impact on yield andproductivity, and enabling the manufacture of product which does notrequire an exceptionally low PMIDA content to be conducted underconditions which afford optimal yield, productivity and AMPA/NMG contentwithout constraints that can otherwise be imposed by operation underconditions which generate the exceptionally low PMIDA product.

In accordance with such embodiments of the invention, glyphosate isproduced in an industrial manufacturing facility which comprises areaction system for the catalytic oxidation of PMIDA in an aqueousmedium. During designated operations within such facility, the processis conducted under conditions effective to consistently produce aglyphosate product having a PMIDA content less than about 0.06 wt. %,basis glyphosate. The glyphosate product as produced under suchconditions is segregated from other glyphosate product that is producedunder other conditions wherein the PMIDA content is greater than 0.06wt. %, typically greater than 0.10 wt. %, more typically greater than0.15 wt. % on a glyphosate basis.

In the practice of such embodiments of the invention, the catalyticreaction system may optionally be operated in accordance with any of thevarious alternatives described above for providing a product reactionsolution containing PMIDA in a proportion less than about 60 ppm byweight, basis glyphosate plus an increment, if any, resulting from PMIDAthat is recycled to the reaction zone. Additionally, or alternatively,the product recovery operation may be operated under added or modifiedpurge conditions to yield one or more glyphosate products containingless than about 0.06 wt. % PMIDA, or ion exchange may be used to removePMIDA from a stream from which glyphosate is crystallized or otherwiserecovered. The designated operations may, for example, comprise acampaign during which all or part of the glyphosate produced in thefacility is generated under conditions effective to provide a low PMIDAcontent. Such a campaign can be of any desired duration, e.g., a week,two weeks, a month or several months, sufficient, e.g., to produce atleast 1500 metric tons, more typically at least about 7500 metric tonsof the low PMIDA product. As part of the method for producing low PMIDAglyphosate on a campaign basis, a manufacturing forecast is preferablyprepared based on projected sales data, and the periods during which allor part of the operations of the industrial facility are dedicated toproduction of low PMIDA glyphosate may be designated on an organizedbasis in view of such forecasts. In the practice of these embodiments ofthe invention, it may be advantageous, for example, to schedule theproduction of low PMIDA glyphosate during startup after a maintenanceturnaround of the reaction system, product recovery system or both; orafter introduction of a fresh catalyst charge. Where there is anypredictable volatility to raw material prices, scheduling can further bedetermined on the basis of such prices.

Where the facility comprises a plurality of batch oxidation reactors forthe conversion of PMIDA to glyphosate, one or more of the plurality ofreactors may be dedicated for a select period of time to operation underconditions effective to produce a product reaction solution containingless than about 250 ppm PMIDA, glyphosate basis.

Where the facility comprises a continuous reaction system for theconversion of PMIDA to glyphosate, the system may typically comprise aplurality of continuous stirred tank reactors (“CSTRs”) in series. Insuch a series of, e.g., two or three CSTRs, the last of the reactors mayfunction as a finishing reactor which operates under terminalconditions, typically at >300 ppm PMIDA, more typically in the range of500 ppm PMIDA, glyphosate basis. In designated operations, theconditions in the final reactor may be modified by increased oxygenflow, higher temperature, etc., to yield a product reaction solutioncontaining <250 ppm PMIDA. Over a campaign necessary to produce someminimum quantity of glyphosate, e.g., at least about 1500 metric tons,preferably at least about 3000 metric tons, more preferably at leastabout 7500 metric tons, the finishing reactor may be dedicated tooperation under such conditions.

To maintain productivity, it may be preferable to achieve the desiredPMIDA content in the product reaction solution by altering conditions inthe entire series of reactors, as by increasing the oxygen flowsthereto. Where the manufacturing facility comprises a plurality ofcontinuous reaction trains, one or more of these may be dedicated to theproduction of low PMIDA glyphosate, either permanently or during a lowPMIDA product campaign, while the remaining reaction trains may beoperated under conditions optimal for producing glyphosate of a higheracceptable PMIDA content.

Allocation of PMIDA Among Plural Grades of Glyphosate

On either a permanent or campaign basis, the processes as illustrated inFIGS. 1 and 2 may also be adapted to produce different grades ofglyphosate product, e.g., one grade that has a PMIDA content less than600 ppm for use in those applications in which it is desirable toutilize a glyphosate product having a low PMIDA content, such as in thepreparation of herbicidal glyphosate compositions for the control ofweeds in genetically-modified cotton crops, and another grade of higherPMIDA content which is quite satisfactory for multiple otherapplications. Generally, the centrifuge wet-cake produced in centrifuge125 has a lower PMIDA content than the wet-cake produced in centrifuge121 (or 137) because the mother liquor from the vacuum crystallizer isless concentrated than the mother liquor from the evaporativecrystallizer, and because no recycle mother liquor stream is introducedinto vacuum crystallizer 109. The PMIDA content of the solid glyphosateacid product removed from the process by centrifuge 121 can be balancedwith the PMIDA content of the salt concentrate exiting the process fromneutralization tank 127 by increasing the fraction of vacuumcrystallizer slurry underflow 117 from the decantation step that isdirected to evaporative crystallizer centrifuges 121 relative to thatwhich is directed to centrifuge 125 and/or by increasing the fraction ofevaporative crystallizer slurry that is directed to centrifuge 137 forproduction of evaporative crystallizer centrifuge wet-cake to beincorporated into the concentrated glyphosate salt solution in saltmakeup tank 127. If desired, the PMIDA content can be unbalanced, and adisproportionately low GI content salt concentrate prepared byminimizing the fraction of vacuum crystallizer slurry 117 directed tocentrifuge 121, and transferring mother liquor from the evaporativecrystallizer circuit to the neutralization tank via mother liquortransfer line 139 and/or by eliminating the fraction of evaporativecrystallizer slurry which is directed to centrifuge 137.

Alternatively, a low PMIDA content solid glyphosate acid product can beprepared by diverting PMIDA to the salt makeup tank 127. In this case, arelatively high fraction of the vacuum crystallizer slurry underflowingthe decantation step is directed to centrifuge 121, and a high fractionof the evaporative crystallizer slurry is sent to centrifuge 137.According to these various process schemes, the process material balancecan be managed to contemporaneously, or indeed simultaneously, toproduce two separate glyphosate products of distinctly differentglyphosate basis PMIDA content.

As a further alternative to the preparation of low PMIDA contentglyphosate product, the product obtained during process startup can besegregated and dedicated for use in glyphosate composition forapplication to and weed control in genetically-modified cotton crops. Bystarting up with water in the evaporators, neutralization tank andprocess storage vessels (not shown), the impact of PMIDA in recyclemother liquor can be avoided immediately after startup, and kept to amodest level during the early portion of the transient period in whichthe product recovery area gravitates to steady state operation.

Further alternative process schemes for allocating residual PMIDA amongtwo or more glyphosate products are described by Haupfear et al. in U.S.Application Publication No. US 2005/0059840 A1, the entire text of whichis expressly incorporated herein by reference.

Whether by sequential operation, segregated operations, or control ofprocess material balance to simultaneously yield different gradeproducts, the processes of the invention can be implemented to yield aplurality of differing grade products, including a low PMIDA producthaving a glyphosate basis PMIDA content typically less than about 1000ppm, preferably less than about 600 ppm, and at least 25% lower than atleast one other, or preferably any other, of such plurality. Moreover,using any one or more of the various process stratagems described above(or below), a low PMIDA product may be produced having a glyphosatebasis PMIDA content that is less than about 1000 ppm, or less than about600 ppm, and at least about 50% lower, or even at least about 75% lower,than the PMIDA content of another of the plurality of products, orpreferably any such plurality.

Conversion and End Point Determination

As noted above, modifications to the oxidation process conditions toachieve an exceptionally low PMIDA content may inevitably involve somepenalty in yield, productivity, catalyst deactivation, cost and/orAMPA/NMG content of the glyphosate product. These penalties can belargely avoided by the alternative of operating the reaction systemunder conditions optimal for the generation of a product reactionsolution having a PMIDA content in the range of from about 300 to about800 ppm, and removing PMIDA by ion exchange in the course of glyphosateproduct recovery. However, the ion exchange process involves its owncapital, operating and maintenance costs. Accordingly, a furtherincremental advantage can be achieved by controlling the operation ofthe oxidation reaction system to generate a product reaction solutionthat meets a desired PMIDA content sufficient to yield an ultimateproduct of the preferred specification, e.g., <600 ppm PMIDA, whileavoiding overreaction that consumes glyphosate and unnecessarilyincreases the AMPA or NMG content of the reaction solution.

Further in accordance with the invention, various methods and systemshave been devised to monitor the conversion of PMIDA and/or thecomposition of the product reaction solution and to identify an endpoint or residence time at which the reaction can be terminated and/orthe product reaction solution withdrawn from the reactor. These include:(i) in-line chromatography; (ii) Fourier transform infra red analysis;(iii) determination of cumulative oxygen consumption and/or timedifferential oxygen consumption; (iv) monitoring oxidation/reductionpotential; (v) monitoring dissolved oxygen concentration in the aqueousliquid reaction medium; (vi) monitoring the oxygen concentration inreactor vent gas; (vii) monitoring the CO₂ concentration in the reactorvent gas; (viii) determining cumulative CO₂ generation and/or analysisof the CO₂ generation profile; (ix) monitoring instantaneous O₂consumption; (x) monitoring instantaneous CO₂ generation; (xi)electrochemical indication of residual PMIDA; (xii) cumulative heatbalance on the reaction system; (xiii) time differential heat generationin the reaction system; and combinations of these techniques.

According to the first of these alternatives, the reaction solution canbe periodically sampled and the sample passed through a chromatographiccolumn, preferably a liquid chromatography column such as a highperformance liquid chromatography (also known as a high pressure liquidchromatography or “HPLC”) column, that is positioned in proximity to thereactor in which the conversion is concluded, i.e., either in a batchreactor, at the exit of a continuous plug flow reactor such as a reactorcomprising a fixed catalyst bed, or in the final of a series of CSTRs.Although the internal pressure of the reactor is sufficient forwithdrawal of a sample by operation of a sampling valve, in someinstances it may be desired to use a metering pump to provide a specimenof defined volume for the HPLC. A filter is provided upstream of theHPLC (or metering pump) for removal of catalyst and any other solids.The sample may optionally be passed through a heat exchanger to cool itto a controlled temperature for passage through the chromatographiccolumn and/or may be diluted to avoid crystallization in the sample orif called for in the chromatography protocol. Conventional detectionmeans well known to the art may be used to determine the PMIDA contentof the sample.

In a batch process, the reaction may be terminated as the desired PMIDAcontent is reached. In a continuous process, the residence time, oxygenflow rate and/or reaction temperature may be adjusted to achieve thedesired conversion. In a continuous reaction system, the reaction rateis believed to be typically limited by gas/liquid mass transfer until arelatively high conversion has been attained, but after that becomeskinetics limited. Since reaction as such is non-zero order in PMIDA, thereaction progressively slows proportionately to residual PMIDA contentas quantitative conversion is approached, irrespective of how muchoxygen is supplied. Thus, the typical behavior of the reaction isexemplified in FIG. 22 which plots the log of the reaction rate vs. thelog of the residual PMIDA concentration. Until high conversion isreached, the consumption of PMIDA is pseudo zero order. When a highconcentration, e.g., in the 95-98% range is reached, the consumption ofPMIDA becomes limited by the reaction kinetics, which are typicallyapproximately first order with respect to PMIDA. Continuous reaction canbe conducted, e.g., in a series of CSTRs as described above, or in aplug flow reactor such as a tubular reactor comprising a fixed orfluidized catalyst bed. In such systems, conversion rates may be mosteffectively increased by increasing oxygen supply along the flow path ofthe reaction medium in the oxidation reaction zone(s) prior totransition to non-zero order conditions. This increases the masstransfer rates and, thus, the reaction rate in the upstream space wheremass transfer controls, and leaves additional reactor space andresidence time for conversion under non-zero order kinetics-limitingconditions to a desired conversion and/or residual PMIDA content.Although FIG. 22 shows the PMIDA consumption rate as substantiallyconstant until it becomes kinetics limiting, it will be understood thatoxygen flow may be reduced toward the end of a batch reaction, or in thepenultimate reactor of a series of CSTRs, which may potentially impactmass transfer coefficients and consequent PMIDA consumption rates,depending on the nature and intensity of mechanical agitation. Oxygenflow rate and temperature may also be adjusted in a batch process asdesired to obtain the target PMIDA content in a prescribed batchreaction cycle.

Although even in-line HPLC involves some lag between sampling andanalytical results, such lag may not significantly compromise control ofthe reaction end point, especially in a continuous process, or in abatch process operated using a noble metal on carbon catalyst. It hasbeen observed that glyphosate is not as susceptible to beingover-oxidized to AMPA where a noble metal on carbon rather than anactivated carbon catalyst is used. It has further been observed thatextended exposure to oxidation reaction conditions does not tend togenerate as much NMG in the presence of a noble metal on carbon catalystas it typically does in the presence of an activated carbon catalyst.

In fact, based on a progression of HPLC data on recently precedingbatches, or taken during recently preceding continuous operations, ithas been observed that the reaction end point, or the appropriatereactor residence time, can be controlled with reasonable accuracy basedon either time alone, or time differential from the point at which thePMIDA conversion is determined by HPLC to have reached a certain level,e.g., 95% or 98%. Moreover, a desired conversion or reaction end pointcan be projected from a series of chromatographic analyses in the mannerdescribed below with respect to the use of FTIR for making suchprojection.

The use of in-line Fourier transform infra red (“FTIR”) analysis formonitoring PMIDA conversion is described fully in U.S. Pat. No.6,818,450 which is expressly incorporated herein by reference. Asdescribed therein, in-line FTIR spectroscopy may be used toquantitatively measure one or more of PMIDA, glyphosate, formaldehyde,formic acid, N-methyl-N-(phosphonomethyl)glycine (“NMG”),N-methylaminomethylphosphonic acid (“MAMPA”) or aminomethylphosphonicacid (“AMPA”) in aqueous mixtures thereof. More preferably, an internalreflectance FTIR method is used for “in-situ” measurement of theinfrared spectrum absorbed by the aqueous reaction solution in orexiting one or more of the oxidation reaction zones or reactors. Theinternal reflectance FTIR method allows for the infrared spectrumabsorbed by the reaction solution to be measured in place by locating asensor probe in, on or in proximity to a process line or reaction vesselso that it is immersed in the reaction solution or positioned on adirect or reflected line of sight to the reaction solution thus allowingthe reaction solution to be directly scanned in substantially real timewithout removing a sample of the solution from the vessel or processline in which it is contained. Advantageously in-situ measurementsprovide real time or substantially real time measurements of thereaction solution.

In general, internal reflectance relates to a process wherein aninfrared light beam is modulated using an interferometer, and themodulated beam is reflected off a sample and returned to a detectorwherein the spectral regions absorbed as well as the intensity of theabsorbance within those regions is determined. One technique forpracticing the internal reflectance method is attenuated totalreflectance (ATR) spectrometry which measures the absorbance in a thinlayer of the sample in contact with the sampling surface of a sensordevice. That is, a sensor probe is placed in direct contact with thesample. A modulated infrared beam is transmitted from the FTIRspectrometer to the sensor probe wherein the beam is transmitted througha sampling surface on the probe such that the beam penetrates into athin layer of the sample in contact with the sampling surface of theprobe and is reflected back into the sensor probe. Significantly, thebeam is modified by the sample due to the absorbance characteristics ofthe sample. The modified beam is then optically transmitted to the FTIRspectrometer's detector. Depending on the ATR probe selected (i.e., theoptical characteristics and geometry of the sampling surface) themodulated infrared beam may reflect off of both the sample layer and thesampling surface a number of times before finally returning back intothe sensor probe, providing additional data to the detector. Thus, ATRprobes are typically described by the number of reflections that occurthrough the sample layer. Preferably, the ATR probe utilizes at leastabout 3, more preferably at least about 6 and still more preferably atleast about 9 reflections or greater.

Preferably, the sampling surface of the ATR probe is comprised ofdiamond. ATR probes comprising a diamond sampling surface may furthercomprise an additional optical element which acts both as a support forthe diamond, and for transmitting and focusing the modulated infraredbeam to and from the diamond sampling surface. Since the second opticalelement is not in contact with the reaction solution, it is lessimportant that the second optical element have the corrosion andabrasion resistance as the sampling surface. Zinc selenide crystals havesimilar optical qualities as diamond at a substantially reduced cost.Accordingly, zinc selenide may be used as an additional optical element.

The sampling surface of the ATR probes may be concave, convex or have arelatively flat surface curvature. Preferably, the sampling surface ofthe ATR probe is relatively flat. Without being held to a particulartheory, it is believed that sampling surfaces having a significantdegree of curvature tend to promote the adherence of particulates suchas catalyst or undissolved product to the sampling surface therebyinterfering with the sensor.

ATR probes having the characteristics described above, i.e., arelatively flat diamond sampling surface are commercially available, forexample, from Axiom Analytical, Inc. (Irvine, Calif.). In a preferredembodiment, a 9 reflection, diamond-composite sensor probe having arelatively flat diamond sampling surface, such as a DICOMPTh SENTINALThATR diamond-composite sensor probe which is commercially available fromASI Applied Systems (Annapolis, Md.), is used.

The FTIR spectrometer detects the intensity or amplitude of the modifiedbeam across the infrared region and transforms the data into anabsorbance spectrum, i.e., absorbance vs. wavenumber. FTIR spectrometerstypically use two types of detectors, a mercury cadmium telluride (MCT)detector or a deuterated triglycine sulfate (DTGS) detector. AlthoughMCT detectors tend to be faster than DTGS detectors and have a highsensitivity, MCT detectors typically require liquid nitrogen cooling.Therefore, it may be preferred to use a DTGS detector which does notrequire liquid nitrogen cooling. Either type of detector may be used.

The reaction solution is typically sampled over a spectral range ofwavelengths from about 2 to about 50 microns, i.e., wavenumbers rangingfrom about 200 to about 5000 cm⁻¹, more preferably from about 650 toabout 4000 cm⁻¹, with wavenumber being the reciprocal of wavelength andproportional to frequency. The infrared spectrum is a continuousspectrum, however for analytical reasons, discrete wavenumbers or groupsof wavenumbers are typically measured. The wavenumber resolution, i.e.,the range of wavenumbers that are grouped together for each discretemeasurement may be increased or decreased to affect the signal to noiseratio of the FTIR spectrometer. That is, as the numerical value of thewavenumber resolution is decreased, more measurements are taken acrossthe spectrum and the resolution of the spectrum increases. However,increases in the wavenumber resolution also typically results in acorresponding increase in the level of “noise”. Generally, FTIRspectroscopy methods use wavenumber resolutions having a numerical valueof 2, 4, 8 or 16, i.e., sample data are collected over discrete rangesof 2, 4, 8 or 16 wavenumbers with the resolution being inverselyproportional to the numerical value of the wavenumber resolution.Typically, a wavenumber resolution of 8 appears to provide a spectrumwith a fairly good resolution while minimizing the amount of “noise.”Changes in the wavenumber resolution may be made, however, withoutdeparting from the scope of the present invention.

Additionally, FTIR spectroscopy generally utilizes a number of scansproviding additional interferometric data, i.e., intensity vs.wavenumber data used in the Fourier transform to produce the spectraldata, i.e., absorbance vs. wavenumber. If the number of scans is set at180, for example, the spectrometer will scan the entire wavelength rangespecified 180 times and produce 180 interferograms, or 180 intensitymeasurements per wavenumber, or more precisely, per wavenumber groupingas determined by the wavenumber resolution. Fourier transforms thencombine the intensity data and convert the 180 interferograms into asingle absorbance spectrum. The number of spectra, i.e., scans can alsoaffect the signal to noise ratio. Generally about 180 scans may besampled with a new spectrum measurement being generated about every 145seconds. More preferably, the number of scans is at least about 360,producing a new measurement every 5 minutes or greater in an effort toimprove the signal to noise ratio.

Preferably, the number of scans performed is such that the frequency inwhich new spectrum measurements are taken is less than about theresidence time of the oxidation reaction zone affecting theconcentration being measured. That is, as described above, the oxidationprocess may utilize one, two or more reaction zones or reactors forconverting PMIDA to glyphosate product. Each reaction zone has acorresponding residence time in which the reaction takes place. Inaddition, if these reaction zones are placed in series, there willadditionally be an overall residence time for the reaction systemcomprising the summation of the residence times for each reaction zone.The residence time considered for determining the sample frequencydepends on whether PMIDA or other analyte is being measured to monitorthe progress of the oxidation reaction in a particular reaction zone, orthe progress of the overall reaction system.

Preferably at least one, more preferably at least two, and still morepreferably at least three measurements are taken within a time period ofno greater than the residence time for the oxidation reaction zone beingmonitored. Typically, the residence time for a particular reaction zonemay vary from about 3 to about 120 minutes, more preferably from about 5to about 90 minutes, still more preferably from about 5 to about 60minutes, and still even more preferably from about 15 to about 60minutes. The residence time for a particular reaction system may varydepending on the total throughput and the quantity of the reactionmixture in the reactor without departing from the scope of the presentinvention.

A single analyte will produce a spectrum having an absorbance profilecharacteristic of that analyte. That is, the spectrum containsabsorbance features that may be associated with the analyte.Accordingly, the concentration of the analyte may be determined using amathematical model representing the relationship between theconcentration of the analyte and the absorbance profile. Themathematical model may be developed by measuring the spectrum for anumber of standard samples having known concentrations andmathematically correlating the concentration as a function of theabsorbance profile using a number of correlation methods. Unfortunately,the characteristic spectrum for a mixture of analytes such as thereaction solution resulting from oxidation of PMIDA is more complex inthat the characteristic absorbance spectrum for the various analytes arebroad and overlap significantly. This overlap precludes the use ofsimple univariate correlation methods for quantitation of the analytesin a reaction mixture. This problem may be overcome by applying morepowerful multivariate mathematical correlation techniques to theanalysis of the spectral data. These multivariate mathematicaltechniques when applied to process chemical analysis are collectivelyreferred to as chemometrics. This technique uses complex mathematicssuch as matrix vector algebra and statistics to extract quantitativeinformation (e.g., concentrations) from highly convoluted orstatistically confounded data such as the spectrum obtained from amixture of analytes to develop a mathematical model, also called achemometric model representing the quantitative information as afunction of the spectrum. A number of multivariate mathematicaltechniques have been developed such as; K-Nearest Neighbors analysis(KNN), Heirarchical Cluster Analysis (HCA), Principal Component Analysis(PCA), Partial Least Squares (PLS) analysis, and Principal ComponentRegression (PCR) analysis. Commercially available software packages arecapable of performing many of the multivariate mathematical correlationtechniques listed above. In fact, at least one commercially availablesoftware package called PIROUETTE (which can be obtained fromInfometrics, Inc., P.O. Box 1528 Woodinville, Wash. 98072) is capable ofperforming all of the correlation techniques listed above.

Commercially available FTIR spectrometers often include chemometricanalysis software. In particular, PLS and PCR are typically used fordetermining a chemometric model, and applying it to a FTIR spectralmeasurement to calculate a property of the sample measured. Of thesetwo, PLS is most commonly applied to FTIR spectral data because itgenerally provides the most accurate chemometric models. PLS allows eachanalyte to be modeled separately, and only requires knowledge of theparticular analyte being modeled. That is, it does not require that theconcentration of each absorbing analyte be known as long as eachabsorbing analyte is represented in the standards used for developingthe chemometric model. Advantageously, the standards can be takendirectly from the process and need not be separately prepared, thusallowing consideration of the impurity profile of the reaction mixturewhen determining the chemometric model for each analyte to be measured.However, it should be noted that the absorbance of the spectral regionsis generally nonlinear with respect to concentrations. Thus, thechemometric models correlating the concentration and the absorbancespectrum should be developed over particular ranges of concentration forthe individual analytes of the reaction solution. That is, the standardsused in the chemometric analysis should be representative of the matrixof concentrations for each analyte in the reaction solution.

In general, therefore, a number of standards are analyzed using the FTIRSpectrometer to measure the spectrum for each standard. Theconcentration of a particular analyte can then be mathematically modeledas a function of the spectra obtained i.e., an algorithm is developedthat correlates the concentration and the spectrum. Although any of themultivariate mathematical calibration techniques may be used, apreferred embodiment uses the PLS method to model the spectra as afunction of concentration. The number of standards used is preferably atleast about 10 and more preferably at least about 20. In general, theaccuracy of the model increases with increases in the number ofstandards used to generate the model. Therefore, the number of standardsused to generate the model may be as high as 50 or greater. Suchstandards may be prepared mixtures, or alternatively, may be samples ofthe particular process mixture to be analyzed. However, as statedearlier, it is preferred that the process mixture is used such that theimpurity profile is considered in the PLS analysis when generating thechemometric model. The concentration of the analyte being modeled ineach standard may be measured off-line using standard analyticaltechniques such as high pressure liquid chromatography (HPLC).Accordingly, chemometric models may be generated using a partial leastsquares regression analysis for spectra obtained from reaction mixturesfrom either a batch or a continuous oxidation process based on-linespectral measurements and off-line HPLC concentration measurements.

As stated earlier, the FTIR scans the reaction solution over a spectralrange of wavelengths corresponding to wavenumbers of from about 200 to5000 cm⁻¹ and more preferably from about 650 to about 4000 cm⁻¹.Although the entire spectral region scanned may be used in the PLSanalysis, generally, the spectral region considered in the PLS analysisis preferably from about 800 to about 1800 cm⁻¹ when modeling the PMIDAsubstrate, glyphosate product, formaldehyde or formic acid analytes.More preferably, however one or more spectral regions selected from thetotal spectrum are considered in the PLS analysis, with the regionsbeing selected based on the analyte to be measured. For example,spectral regions to be considered in the PLS analysis may be selected byidentifying spectral regions of characteristic peaks for each analyte ina solute such as water. Preferably however, the spectral region used inthe PLS analysis to develop a chemometric model for PMIDA is from about800 to about 1450 cm⁻¹, and more preferably from about 1065 to about1400 cm⁻¹. The spectral region or regions used in the PLS analysis todevelop a chemometric model for glyphosate in the reaction solution arepreferably from about 800 to about 1450 cm⁻¹, and more preferably boththe region from about 865 to about 945 cm⁻¹ and the region from about1280 to about 1460 cm⁻¹. The spectral region used in the PLS analysis todevelop a chemometric model for formaldehyde is preferably from about800 to about 1450 cm⁻¹, more preferably from about 945 to about 1150cm⁻¹, still more preferably from about 945 to about 1115 cm⁻¹, and stillmore preferably from about 1000 to about 1075 cm⁻¹. Finally, spectralregion or regions used in the PLS analysis to develop a chemometricmodel for formic acid is preferably from about 800 to about 1450 cm⁻¹,more preferably the region(s) from about 1150 to about 1300 cm⁻¹ and/orfrom about 1650 to about 1800 cm⁻¹. While the preferred spectral regionsfor formic acid provide reasonable accuracy at higher concentrations offormic acid, i.e., around from about 2,000 to about 5,000 ppm formicacid, the accuracy of the model decreased significantly at lowerconcentrations, i.e., less than about 1,000 ppm or even less than about600 ppm. Significantly, a strong absorption band exists within theformic acid spectral region at around 1721 cm⁻¹. This band is close tothe 1600 cm⁻¹ water region which, for aqueous mixtures, is subtractedout as a background and thus can be inconsistent and difficult toquantify. Thus, to minimize the effects of the water subtraction, thepreferred spectral region used in the PLS analysis to develop achemometric model for low concentrations of formic acid is preferablyfrom about 1710 to about 1790 cm⁻¹. Surprisingly, by avoiding thespectral region which overlaps the water region, the present inventionprovides quantitative measurement of formic acid at concentrations lessthan about 1,000 ppm, less that about 600 ppm and even less than about300 ppm.

Using the PLS analysis techniques therefore, chemometric models used todetermine the concentration of PMIDA, glyphosate, formaldehyde, and/orformic acid analytes as a function of the absorption spectrum may bedeveloped and used in combination with the FTIR spectrometer to providereal-time concentration data for process mixtures from either a batch ora continuous process thus allowing for improved studies of the reactionkinetics, improved reaction control, and in the case of the batchprocesses, a more accurate and timely reaction end point determinationto be made.

For example, using the techniques described above, chemometric modelshave been developed using an FTIR spectrometer and a diamond-compositeATR probe such that the concentration of PMIDA in a reaction solutionmay be measured over a range of concentrations of from about thedetection limit, currently about 50 ppm, to about 4% with a PLS meanerror of less than about 0.2% for a batch oxidation process and may bemeasured over a range of concentrations of from about 200 ppm to about4,500 ppm with a mean error of about 200 ppm for a continuous oxidationprocess. The concentration of glyphosate product in a reaction solutionmay be measured over a range of concentrations of from about 5% to about10% with a mean error of less than about 0.2% for batch oxidationprocesses and may be measured over a range of concentrations of fromabout 4% to about 8% with a mean error of about less than about 0.2%,more preferably less than about 0.07% for continuous processes. Theconcentration of formaldehyde in a reaction solution may be measuredover a range of concentrations of from about 130 ppm to about 6,000 ppmwith a mean error of less than about 150 ppm for batch processes and maybe measured over a range of concentrations of from about 250 ppm toabout 4,500 ppm with a mean error of about less than about 55 ppm andeven over a range of concentrations of from about 100 ppm to about 400ppm with a mean error of less than about 50 ppm and preferably less thanabout 30 ppm for continuous processes. Finally, the concentration offormic acid may be measured over a range of concentrations of from about0.3% to about 1.3% with a mean error of less than about 0.03%,preferably less than about 0.02% for batch processes and may be measuredover a range of from about 0.1% to about 0.4% with a mean error of aboutless than about 0.02%, more preferably less than about 0.01% forcontinuous processes.

As described in U.S. Pat. No. 6,818,450, in response to the measurementsmade by in-line FTIR analysis, various adjustments may be made to eithera batch or continuous oxidation reaction system as are discussed abovewith respect to in-line HPLC. More particularly, the FTIR analyticalmethods described above may be used to measure the progress or conditionof the reaction solution resulting from the oxidation of a PMIDAsubstrate to form a glyphosate product by providing substantially realtime concentration analysis for PMIDA or one or more other analytes. Inresponse to the substantially real time measurement, one or more processeffects may be controlled by adjusting or maintaining the value of oneor more independent process variables affecting the rate of oxidation ofthe PMIDA substrate, the rate of oxidation of formaldehyde, the rate ofoxidation of formic acid, the rate of oxidation of glyphosate product toaminomethylphosphonic acid or salt or ester thereof. Independent processvariables affecting the rate of oxidation of the PMIDA substrate, therate of oxidation of formaldehyde, the rate of oxidation of formic acid,the rate of oxidation of glyphosate product to aminomethylphosphonicacid (or salt or ester thereof) include but are not necessarily limitedto: the rate of introduction of molecular oxygen into the continuousreaction zone, the rate of withdrawal of gas from the reaction zone, theoxygen partial pressure at a select location within the reaction zone orin contact with the liquid reaction medium, the temperature of thereaction mixture, the rate of introduction of the aqueous feed mixtureto the reaction zone, the rate of withdrawal of the reaction solutionfrom the reaction zone, the amount of catalyst added to the reactionzone, the amount of catalyst removed from the reaction zone, the amountof a supplemental catalyst promoter added to the reaction zone and theintensity of agitation of the reaction mixture.

For example, the oxidation reaction may be carried out in a batchprocess. An internal reflectance sensor, preferably an ATR probe isinserted directly into the reactor, or alternatively is placed in-linewith a recycle loop to enable in-situ real time or substantially realtime measurements of the concentration of at least one of the analytesin the reaction mixture. The progress of the reaction can be determinedby monitoring the decrease in the concentration of the PMIDA substrate,for example, or alternatively by monitoring the increase in theconcentration of the glyphosate product, thus enabling real time orsubstantially real time determinations of the reaction endpoint. Inaddition, the data from the FTIR may be electronically communicated to aconventional process control apparatus. Preferably, the processcontroller is configured such that in response to the data showing theend point of the reaction had been reached, the process controllerinstructs a control device such as a control valve to terminate theintroduction of the oxygen-containing gas into the reaction zone(s) suchthat the oxidation reaction is terminated. It should be noted that theabove example is for illustrative purposes only and in no way isintended to limit the manner in which the progress of the batchoxidation process or the condition of the reaction mixture therein iscontrolled in response to the analyte concentration measurement providedby the FTIR analytical method.

In another embodiment, FTIR analysis is used to monitor a PMIDAoxidation process conducted in a continuous fashion in two or more CSTRsin series as described above and, for example, in U.S. Pat. No.7,015,351, the entire contents of which are incorporated herein byreference. For example, the concentration of unreacted PMIDA, glyphosateproduct and/or oxidation by-products in the reaction mixture effluentare measured using the analytical method described above. In aparticularly preferred embodiment of the present invention, theconcentration of unreacted PMIDA, glyphosate product and/or oxidationby-products in the intermediate aqueous reaction mixture withdrawn fromthe first stirred tank reactor and/or in the final reaction mixtureeffluent withdrawn from the second or subsequent stirred tank reactormay be measured using the analytical method described above. Based onthese and other process measurements, control adjustments can be made,thus the conversion of the PMIDA substrate and condition of the reactionmixture may be controlled by controlling the total oxygen feed to thecontinuous oxidation reactor system, i.e., each of the stirred tankreactors and/or the apportionment of the total oxygen feed between thetwo or more CSTRs and may be adjusted to beneficially affect the yieldand quality of the glyphosate product. Alternatively, other variablesmay be controlled such as the partial pressure of oxygen at a selectlocation within one or more of the reaction zones or in contact with theliquid reaction medium of each reaction zone, the rate of withdrawal ofgas from one or more of the reaction zones, the temperature of theliquid reaction medium within one or more of the reaction zones orexiting one or more of the reaction zones, the rate of withdrawal ofreaction product solution from one or more of the reaction zones, theliquid level of reaction mixture in one or more of the reaction zones,the weight of reaction medium in one or more of the reaction zones,addition or removal of catalyst to the oxidation reaction system via oneor more of the reaction zones, shifting the relative proportions of thetotal catalyst mass in one or more of the reaction zones as well as acatalyst holding tank, the addition of a supplemental catalyst promoterto one or more of the reaction zones and the intensity of mixing in oneor more of the reaction zones.

Moreover, in making adjustments of control variables in response to FTIRanalysis, other process effects may be taken into account, e.g., theoxygen content of the gas withdrawn from the reaction zone(s), dissolvedoxygen in the liquid medium in the reaction zone(s), the response of anoxygen electrode or voltage of an oxidation/reduction potentialelectrode, and the noble metal content of the liquid phase of thereaction mixture withdrawn from the reaction zone. By considering thesetogether with current values for control variables and real time FTIRanalysis of the concentrations of one or more analytes in the reactionmixture(s), one or more control variables may be adjusted to conform theprocess to established process constraints and/or to optimizeeconomically significant outputs such as yield, conversion, selectivity,by-product content and process emissions. With the benefit ofsubstantially real time analysis of reaction mixture composition,optimization can be determined either ad hoc based on known processperformance relationships, or in accordance with protocols that havebeen established based on such relationships. As appropriate, materialbalance, energy balance, kinetic, mass transfer, heat transfer, thermalstability, catalyst deactivation profiles, and other conventionalconsiderations can form the basis for establishing protocols. As may beconvenient, such protocols may optionally be reduced to algorithms whichcan be programmed onto a processor. Assimilating additional information,including both control variables and performance measurements such asthose described above, the processor may then determine optimum settingsfor one or more of the aforesaid independent variables in accordancewith the protocol for obtaining a desired or optimal value for theconcentration of one or more of the analytes with respect to an economicor process criterion selected from the group consisting of conversion ofsubstrate, yield of said product on said substrate, selectivity of theoxidation reaction for the glyphosate product, quality of productrecoverable from the reaction mixture, productivity, emissions inprocess effluents, stability of catalyst activity, and manufacturingcost.

To improve determination of residual PMIDA at the conclusion of a batchcycle, or under terminal conditions in a continuous back mixed reactionsystem, the chemometric models for refining the FTIR analysis of thevarious components of the reaction solution may be integrated withmaterial balance computations, energy balance computations and othermeasured process data to further enhance the precision and accuracy ofPMIDA determination.

In a particularly advantageous application of FTIR, the instantaneousrate of depletion of PMIDA during non-zero order reaction conditions maybe used to aid in determining the instantaneous PMIDA concentration. Asdiscussed in more detail below in connection with methods based onoxygen consumption, carbon dioxide generation and heat generation, theorder of the PMIDA oxidation reaction, the order of reactions by whichformaldehyde is oxidized to formic acid and formic acid is oxidized toCO₂ and water, and the kinetic rate constants for the depletion of PMIDAand oxidation of by-products such as formaldehyde and formic acid, maybe estimated from historical analytical data, or historical operationaldata as obtained from laboratory and/or industrial oxidation reactions.As further explained below, the estimate of the kinetic rate constantsmay also be refined by reference to current operational data, includingthe observed rate of decline in the rate of reaction. Whereas themeasure of reaction rate and decline thereof is indirect in the case ofoxygen consumption, carbon dioxide generation or heat generation, FTIRprovides a direct measurement of residual PMIDA, formaldehyde and formicacid concentrations. Thus, a chemometric model based on FTIR allowsreaction material balance, energy balance and kinetic reaction ratecomputations to be integrated with direct quantitative measurement ofresidual PMIDA in projecting an end point of the reaction. Duringnon-zero order reaction when the rate of reaction is declining, asequence of two or more FTIR measurements may provide a reliable basisfor projecting the time at which an end point corresponding to a desireddegree of PMIDA conversion (and residual PMIDA concentration) may beachieved. In a batch reaction, the separate analyses of the series aretaken at different times during the course of the reaction, withpreferably at least two of these being taken during the non-zero orderreaction period approaching the end of the batch. In a continuousreaction system, the samples may be taken at differing residence times,again preferably in a non-zero order regime. In these circumstances, therate of the reaction declines from analysis to analysis as a function ofresidual PMIDA content which is defined by the order of the reaction,affording a basis for projecting the time by which a desired end pointconcentration of PMIDA will be attained. To the extent that the reactionapproximates first order in PMIDA, the projection may be made bystraight line extrapolation on a plot of the logarithm of concentrationvs. time.

The order of the reaction and the kinetic rate constant may bedetermined from a plurality of analyses sufficient to define the contourof the relationship between reaction rate and residual PMIDAconcentration. In a plot of residual PMIDA (reactant) concentration vs.time (or vs. distance in a plug flow reactor), as illustrated in FIG. 5,the first derivative at any given time or location representsinstantaneous reaction rate at that time or location. In a plot of thelog of the reaction rate vs. the log of the residual PMIDA (reactant)concentration, as illustrated in FIG. 6, the slope indicates the orderof reaction and the appropriate intercept indicates the rate constant(or, more directly, a pseudo rate constant combining the kinetic rateconstant, a dissolved oxygen term and appropriate mass transfereffects).

In a continuous reaction system, FTIR is applied directly fordetermining conversion, but control can be refined by projectingconversion from FTIR analyses of the reaction medium at a plurality ofdifferent residence times. In a flow reactor (e.g., plug flow reactor),analyses at differing residence times may be obtained by applying FTIRto the reaction medium at separate points along the flow path of thereaction medium. Preferably, at least two of such analyses are at pointswithin the zero order regime. Based on a known or determined order ofreaction, this allows conversion and residual PMIDA content to beprojected, and to be controlled in a predictable manner by adjustment offeed rate, oxygen flow, and/or reaction temperature. In a series ofCSTRs, the last reactor operates under terminal conditions, whichtypically reflect a conversion >95% and are, therefore, ordinarilynon-zero order with respect to PMIDA. By comparison of the FTIR analysisof the reaction medium at two different residence times, e.g., where oneanalysis is made at the exit of the final reactor and another at theexit of the penultimate reactor at steady state, the reaction rateconstant can be inferred based on a known or determined order ofreaction. The estimate of the rate constant can be refined by separateanalyses of samples from the two reactors by laboratory or in-line HPLC.Conversion may thereafter be projected based on continual or repetitiveFTIR analyses of the reaction medium exiting the penultimate reactor.

Although FTIR data obtainable during non-zero order reaction conditionsare particularly useful in estimating the order and rate of theoxidation reactions, the methods of the invention encompass applicationof algorithms by which end points are projected from data taken underthe zero order or pseudo zero order conditions that typically prevailduring more than 95% of a typical batch reaction cycle, or in CSTRsother than the last of a series thereof, or in most of the length orheight of a plug flow reactor. As further discussed below, laboratoryand plant data may be combined to provide a general algorithm forprediction of end point from data taken at various points in the batchreaction cycle or various positions in a continuous reaction train.

As also discussed below, FTIR analysis can be combined with othermethods for monitoring the oxidation reaction to refine the estimate ofconversion, e.g., by providing data by which the estimate provided byother methods can be compensated for oxidation of C₁ by-products.

Cumulative oxygen consumption may provide a further basis for estimatingthe extent of conversion of PMIDA. As noted above, the oxidation reactoris preferably operated under pressure control, with the head space ofthe reactor being vented in response to a pressure sensor to maintain asubstantially constant pressure. By measuring the rate of introductionof an oxygen-containing gas of known O₂ content, measuring the rate atwhich the vent gas is removed from the oxidation reaction zone, andanalyzing the vent gas for oxygen, the instantaneous rate of oxygenconsumption can be determined. By integrating the oxygen consumptionrate over time, cumulative oxygen consumption can be determined; and thecumulative oxygen consumption is substantially proportional to PMIDAconversion, as adjusted for formaldehyde and formic acid formation andoxidation, i.e., oxygen consumption is stoichiometrically equivalent toPMIDA conversion and formaldehyde and formic acid formation, accordingto the relationship:

Cumulative oxygen consumption can be compared to the initial PMIDAcharge to a batch reactor or to the time integrated rate of PMIDAintroduction of a continuous reactor. Adjustment for the generation andoxidation of formaldehyde and formic acid can be made on the basis ofon-line analysis for these compounds, or on the basis of historicaldata. The historical value may be either a fixed figure based on longterm statistical analysis of analytical data on the product reactionsolution, or an evolutionary value based on statistical analysis ofrecent historical data, either of recent batches in a batch oxidationprocess or recent analyses at comparable residence time in a continuousprocess, in each case under temperature, oxygen flow, oxygen pressure,catalyst charge, etc., which are the same as the batch for whichconversion is being estimated. Where a carbon catalyst is used, theformaldehyde and formic acid values are relatively consistentirrespective of catalyst activity, in part because carbon by itself issignificantly less effective than a noble metal on carbon catalyst foroxidation of these by-products. A noble metal on carbon catalyst is muchmore effective for oxidation of C₁s, so that the condensed phase C₁content is more dependent on catalyst activity as affected, e.g., bycatalyst age and replenishment.

A continuous reactor may also be controlled to maintain a targetedinstantaneous ratio of oxygen consumption to PMIDA introduction. Theconversion as estimated from cumulative oxygen consumption over adefined period of operation may further be adjusted by any difference inthe molar rate at which PMIDA is introduced into the reaction system vs.the molar rate at which the sum of glyphosate and unreacted PMIDA arewithdrawn therefrom. Preferably, however, steady state operation ismaintained both to preserve process stability generally, and toeliminate any accumulation or decline in working reaction volume as avariable affecting the estimate of conversion.

Where conversion is monitored by oxygen consumption, estimatedconversion can be refined on a continuous or repetitive basis by use ofan oxygen electrode, which effectively measures the dissolved oxygencontent of the aqueous reaction medium, and/or by an oxidation/reductionpotential probe, which measures the potential of the catalyst in themedium. Typical traces for O₂ flow and dissolved oxygen content of theaqueous medium as a function of time are shown for a typical batch cyclein FIG. 18. Where the rate of oxygen introduction is controlled inresponse to the oxygen electrode to maintain a constant dissolved oxygenlevel, it may contribute to the precision of the correlation betweenoxygen consumption and conversion.

The accuracy with which oxygen consumption is used to estimateconversion and/or residual PMIDA content can be enhanced by combiningmeasurement of oxygen consumption with other methods for determiningconversion. For example, a base point PMIDA content can be determinedanalytically from a sample taken at a relatively high conversion, e.g.,in the neighborhood of 95%, and the oxygen consumption measured from thetime at which the base point sample is taken. In this manner, error inmeasurement of cumulative oxygen flow, or arising from consumption ofoxygen other than for oxidation of PMIDA, or PMIDA and C₁ by-products,is a fraction only of the incremental oxygen requirement for the finalstage of conversion rather than a fraction of the total oxygen requiredfor conversion of all PMIDA charged to the reactor. Thus, for example,if the target conversion is 99.0%, and an FTIR or HPLC analysis ofsample taken to establish a base point late in the batch indicates aconversion of 94.7%, a 3% error in correlating measured cumulativeoxygen consumption with conversion from the base point forward equatesto only a ˜0.13% error in overall conversion in addition to whatever theerror may be in the base point sample analysis.

Such combination of measurements is particularly advantageous because itproceeds from a base point at which residual PMIDA content remainsappreciable. Up to this point, the PMIDA analysis remains quitereliable, but cumulative oxygen consumption is subject to significanterror. The method then combines a chemical analysis at the base pointwith determination of oxygen consumption during the final stage of thereaction subsequent to that base point, during which accuracy ofresidual PMIDA content may typically begin to deteriorate but error indetermination of oxygen consumption contributes minimal error to theoverall determination. Stated another way, the method is governed byanalytical results during the major portion of the reaction in whichchemical analysis is the most reliable measure, but switches to oxygenconsumption during the closing stages of the reaction wherein oxygenconsumption reliability not only is relatively enhanced, but typicallyalso becomes superior to the reliability of chemical analyses.

The method can further be refined by analytical measurement of residualformaldehyde and formic acid throughout the reaction, and particularlysubsequent to the high conversion base point. Measurement of any changesin formaldehyde and formic acid content between the base point and anactual or test end point allows the relationship between oxygenconsumption and residual PMIDA content to be refined by compensating forthe oxygen that may be consumed in the oxidation of the C₁ by-products.Although the accuracy of analytical methods such as FTIR for PMIDAdeteriorates at high conversions, the accuracy of analyses offormaldehyde and formic acid remains quite high. Moreover, becauseformaldehyde and formic acid are being both consumed and generatedduring the reaction, including the final stage subsequent to the basepoint, the residual level of residual C₁ compounds is typically greaterthan that of residual PMIDA at the desired end point, furthercontributing to the accuracy of analyses for these by-products by FTIR,HPLC or other appropriate technique.

Practice of the combined method with C₁ compensation is furtherelaborated below with respect to the heat generation method forassessing conversion and estimated reaction end point. The reaction canbe monitored after the high conversion base point by either oxygenconsumption, heat generation or CO₂ generation. C₁ compensation afterthe base point operates on substantially the same principle in all ofthese methods, with further particulars being provided below inconnection with heat generation.

The declining rate of oxygen consumption at low PMIDA concentrations mayprovide another or further basis for refining the estimate based oncumulative oxygen consumption, and/or afford an independent basis forestimating the residual PMIDA content toward the end of a batch reactioncycle or in the last of a series of CSTRs. The oxidation of PMIDA toglyphosate is fundamentally first order in PMIDA, or approximately so.During the bulk of the reaction, where PMIDA content is high, thereaction rate is limited by mass transfer of oxygen to the aqueousphase. However, towards the end of the reaction, typically at PMIDAconversions in excess of 98%, kinetics become limiting and first orderbehavior is observed. The oxidations of formaldehyde to formic acid andformic acid to carbon dioxide are also non-zero order reactions. Theproper exponents for a particular reaction system in a given range ofconversion can be derived for each substrate, i.e., PMIDA, formaldehydeand formic acid by empirical observation, kinetic studies andstatistical analysis. The kinetic rate constants for each reaction canbe derived from a combination of laboratory data and industrial reactordata comparing oxygen consumption with PMIDA, formaldehyde and formicacid analyses of a series of samples taken during non-zero orderoperations, i.e., under conditions in which the reaction rate isdeclining. From data establishing the order of the respective reactionsand applicable rate constants, the residual PMIDA content may beinferred from a comparison of the initial batch PMIDA charge, or rate ofintroduction of PMIDA into a continuous reaction system, vs. theresidual rate of oxygen consumption at substantially constant dissolvedoxygen concentration during non-zero order reaction, either at the endof a batch reaction cycle or in the final of a series of CSTRs. To theextent that the behavior of the several oxidation reactions approximatesfirst order, a component of the instantaneous oxygen consumption rate isdirectly proportional to the residual PMIDA content, a further componentis directly proportional to residual formaldehyde, and a still furthercomponent is directly proportional to residual formic acid. If thereactions are essentially first order and the respective rate constantsare known, a relatively simple algorithm may be developed fordetermining residual PMIDA content as a function of instantaneous oxygenconsumption. To the extent that the order of any of the variousreactions differs from first order, the determination of residual PMIDAcontent from instantaneous oxidation rate becomes more complex. However,where the orders of the reactions are reasonably established fromhistorical laboratory and/or industrial scale data, rigorous equationsand/or statistical correlations can be developed by which to sort outresidual PMIDA from the instantaneous oxygen consumption rate. In thecase of a series of CSTRs, the differential oxygen consumption ratemethod may be calibrated by estimating the kinetic rate constant orfunction thereof from PMIDA analysis of samples of the aqueous mediumentering and exiting the final reaction zone. At either conventionalconversions or in the production of a product reaction mixture ofexceptionally low PMIDA content, the reaction occurring in the finalstage reaction zone of a continuous oxidation process is ordinarilynon-zero order. As in the case of C₁ compensation as discussed above,the historical data may be based either on long term statisticalanalysis, or reflect evolutionary values based on analysis of recentperformance. Sampling may also provide the basis for estimating thekinetic rate constants for the oxidation of formaldehyde and formicacid. Based on data defining the orders of the various reactions and therespective rate constants, oxygen consumption associated with C₁oxidation can also be estimated in a continuous system, allowingresidual PMIDA to be determined from the balance of the oxygenconsumption.

Once a base line has been established, the value of the rate constantcan be adjusted based on variation in the rate of decline of oxygenconsumption from the point at which non-zero order behavior is observed,i.e., from the (negative) first derivative of oxygen consumption rate,the absolute value of which is inversely related to the rate constant.Also, as the catalyst ages and its activity significantly declines, theeffect on first order rate constants can be periodically re-calibratedby renewed sampling of the final reaction zone, or near the end of thebatch. See the derivation set forth below for determination of the rateconstant by analysis of declining reaction rate as measured by heatgeneration. The same analysis is applicable to oxygen consumption,substituting this term for heat generation in the derivation.

In a batch reaction, a function of the rate constant and the order ofthe reaction may also be established from a plurality of measurements ofinstantaneous oxygen consumption, substantially as described above withrespect to FTIR. If instantaneous oxygen consumption is monitored incombination with analyses of the reaction solution, e.g., using FTIR orHPLC, and the actual residual PMIDA content determined as a function ofinstantaneous oxygen consumption at one or more points during thenon-zero order oxidation stage, plural measurements of instantaneousoxygen consumption during this stage may be used to project a desiredend point of a batch reaction, also as described above with respect toFTIR. Conversion in the final CSTR of a series thereof can be projectedbased on measurement of instantaneous oxygen consumption relative toPMIDA feed rate in the reactors upstream of the final reactor, andresidence time in the final reactor. In this instance, the order of thereaction may be separately determined from historical analytical oroperational data obtained from a laboratory or industrial batch reactor.

As further discussed below, oxygen consumption can be combined withother methods for monitoring the reaction to refine the estimate ofconversion. For example, HPLC, FTIR, or electrochemical oxidationanalyses can be used to compensate for consumption of oxygen for C₁oxidation.

A further alternative for estimation of PMIDA conversion comprisesmeasurement of cumulative carbon dioxide generation. In the oxidationreaction, one carboxymethyl group is removed and converted to carbondioxide or a combination of carbon dioxide and other C₁ compounds, i.e.,formaldehyde and formic acid. Thus, PMIDA conversion is directlyproportional to the molar sum of cumulative CO₂ generation plusformaldehyde and formic acid generation and oxidation. By measurement ofcumulative CO₂ generation and adjustment for other C₁ compounds, anestimate can be made of the conversion of PMIDA. Adjustment forformaldehyde and formic acid obtained in the reaction can be determinedon essentially the basis described above for estimation of PMIDAconversion by oxygen consumption. Also as in the case of oxygenconsumption, the conversions estimated from cumulative heat generationin a continuous reaction system may be adjusted by any difference in themolar rate at which PMIDA is introduced into the reaction system vs. themolar rate at which the sum of glyphosate and unreacted PMIDA arewithdrawn therefrom.

Further as described above with respect to cumulative oxygenconsumption, the accuracy with which cumulative carbon dioxidegeneration is used to estimate conversion and/or residual PMIDA contentcan be enhanced by combining measurement of CO₂ generation with othermethods for determining conversion. For example, a base point PMIDAcontent can be determined analytically from a sample taken at arelatively high conversion, and CO₂ generation measured from the time atwhich the base point sample is taken. As in the case of determiningconversion and/or end point from oxygen consumption, error inmeasurement of cumulative CO₂ release, or arising from CO₂ generationother than from oxidation of PMIDA, or PMIDA and C₁ by-products, is afraction only of the incremental CO₂ generation during the final stageof conversion subsequent to the base point rather than a fraction of thetotal CO₂ generated in the conversion of all PMIDA charged to thereactor.

This combined method enjoys the same advantages as the combinedanalytical and oxygen consumption method as described above. Thus, it isgoverned by chemical analysis up to the high conversion base pointduring which such analysis is the most reliable, then switches at thebase point to cumulative CO₂ over the final stage of the reaction,during which the latter method typically provides accuracy superior tothat of chemical analysis.

In this combined method, compensation for formation and consumption offormaldehyde and formic acid can be accomplished in the same manner asgenerally described above with respect to the oxygen consumption method,and elaborated below with respect to the heat generation method.

Also, as in the case wherein conversion is estimated from cumulativeoxygen consumption, the estimate based on cumulative CO₂ generation maybe refined, or the residual PMIDA content independently monitored, bymeasuring the declining CO₂ generation rate during the closing portionof a batch reactor, or under the terminal conditions prevailing in thelast of a series of CSTRs. If the rate of oxygen addition is controlledto maintain a constant oxygen potential in response to an ORP electrode,and/or a constant dissolved oxygen content in response to an oxygenelectrode, the accuracy of the estimate based on CO₂ generation isenhanced in essentially the same way that the accuracy of an estimatebased on oxygen consumption is enhanced. FIG. 20 illustrates typicalprofiles of oxygen content and carbon dioxide content in the vent gas asa function of time during the batch catalytic oxidation of PMIDA toglyphosate; and FIG. 21 shows similar profile for oxidation/reductionpotential during the batch.

Since the reaction is first order in PMIDA, the residual PMIDA contentmay also be inferred from a comparison of the initial batch PMIDAcharge, or rate of introduction of PMIDA into a continuous reactionsystem, vs. the residual instantaneous rate of carbon dioxidegeneration, especially at constant dissolved oxygen concentration,either at the end of a batch reaction cycle or in the final of a seriesof CSTRs. For purposes of this alternative, the kinetic rate constant ora function thereof may be estimated and updated in essential the samemanner as is described above for the estimation of PMIDA conversion fromthe rate of decline in the consumption of oxygen. More particularly, therate constant can be determined according to the derivation set forthbelow with respect to heat generation, but substituting carbon dioxidegeneration for heat generation.

In a batch reaction, a function of the rate constant and the order ofthe reaction may also be established from a plurality of measurements ofinstantaneous CO₂ generation, substantially as described above withrespect to oxygen consumption and FTIR. If CO₂ generation is monitoredin combination with analyses of the reaction medium, e.g., using FTIR orHPLC, and the actual residual PMIDA content determined as a function ofinstantaneous CO₂ generation at one or more points during the non-zeroorder oxidation stage, plural measurements of instantaneous CO₂generation during this stage may be used to project a desired end pointof a batch reaction, also as described above with respect to FTIR.Conversion in the final CSTR of a series thereof can be projected basedon measurement of instantaneous CO₂ generation relative to PMIDA feedrate in the upstream reactors and residence time in the final reactor.In this instance, the order of the reaction may be separately determinedfrom historical analytical or operational data obtained from alaboratory or industrial batch reactor.

As further discussed below, carbon dioxide generation can be combinedwith other methods for monitoring the reaction to refine the estimate ofconversion. For example, HPLC and FTIR analyses can be used tocompensate for generation of CO₂ from C₁ oxidation.

In a further alternative embodiment, PMIDA conversion can be estimatedfrom a combination of the oxygen consumption and carbon dioxidegeneration. Although each independently provides a basis for estimation,each may also be used as a check against the other. Moreover,observations of oxygen consumption, carbon dioxide generation, FTIRanalysis, and other parameters may be integrated into a chemometricmodel which also integrates other data relationships such as, forexample, material and energy balance computations for the reaction step.Such a model can also optionally integrate data yielded by other methodsof end point detection as described hereinbelow, including cumulativeheat generation, differential heat generation, and electrochemicaloxidation.

As illustrated in FIG. 20, the oxygen content of the reactor vent gastypically increases rather sharply as the end point of a batch oxidationreaction cycle is approached. Data underlying the profiles illustratedin FIG. 20 provide the basis for determining both cumulative anddifferential oxygen consumption in the oxygen consumption methods of theinvention for estimating end point. In addition, given the strongresponse of vent gas O₂ content to conversion as the reaction nears itsend point, the instantaneous vent gas O₂ content as such, or the rate ofchange therein, provides a reasonably precise basis for projecting areaction end point, or estimating the extent of conversion, irrespectiveof whether the measured O₂ content is converted to a cumulative oxygenconsumption or an instantaneous rate thereof. Historical analytical oroperational data obtained from laboratory or industrial oxidationreactors may be used to calibrate detection of the reaction end point bymeasuring vent gas O₂ content. Conversions and end points as estimatedfrom vent gas oxygen profiles can provide a basis for adjusting reactionparameters such as PMIDA residence time, reaction temperature, andagitation intensity in establishing and maintaining a target residualPMIDA content in the product reaction solution exiting a continuous backmixed or continuous plug flow reactor.

As illustrated in FIG. 20, vent gas O₂ may be relatively high early inthe course of a batch reaction, typically before the aqueous reactionmedium has become heated to the target reaction temperature. In suchinstance, the target end point or conversion is indicated by vent gas O₂content as attained after an appropriate minimum batch reaction time orminimum continuous reactor residence time. The requisite minimum time isthat which is sufficient so that, under the conditions prevailing in thereaction zone, there is a unique correlation between O₂ content of thevent gas vs. conversion and/or residual PMIDA content in the course offurther reaction subsequent to such minimum reaction time or residencetime. For example, the vent gas can be monitored after the reaction hasentered the non-zero order stage with respect to consumption of PMIDA,typically at a conversion above about 95%, or even above 98%. However,the “minimum time” requirement can often be met earlier. Under typicalreaction conditions, the vent gas O₂ content may become a uniquefunction of conversion after a minimum reaction or residence time thatprovides a conversion somewhat below 95%, in which case it may beconvenient to begin following vent gas O₂ content at a such lowerconversion.

In a typical tank reactor, whether batch or CSTR, the head space issubstantially back mixed, and the volume and residence time in the headspace can be substantial, thus potentially damping the response of thevent gas O₂ content to conversion if measured by sampling the bulk gasphase, and consequently tending to mask the end point. Thus, in certainpreferred embodiments, the response is enhanced by monitoring the O₂content of gas phase instantaneously as it exits or evolves from theliquid phase. This may typically be accomplished, e.g., by segregating arepresentative sample of the aqueous liquid reaction medium, anddirecting the sample to a gas/liquid separator operating at the samepressure as the reactor, whence the separating gas phase is analyzed forO₂ content. The sample is withdrawn from below the liquid level and thegas/liquid separator may be vented back into the head space of thereactor. Alternatively, an analytical probe may be immersed in theliquid phase which is effective for sensing nascent gas released fromthe liquid phase. In yet another alternative, a sampling device may bepositioned to capture bubbles forming in the liquid phase and direct itto a gas chromatograph or other analytical device for determining theoxygen content.

Where a noble metal catalyst is used for the reaction, the oxygen flowrate may in some cases be stepped down significantly as the reactionapproaches its end point, e.g., toward the end of a batch reaction cycleor in the final reactor in a series of CSTRs. This creates conditionsthat are conducive to the oxidation of C₁ by-products such asformaldehyde and formic acid, helps prevent oxidation of glyphosate toAMPA, and inhibits oxidative degradation of the catalyst. Where suchmeasures are taken, the instantaneous O₂ content of the vent gas may notincrease as sharply as may be desired for precise end pointidentification. However, even at reduced oxygen flow, the oxygenutilization may drop relatively sharply as the batch approaches its end.Thus, end point can also be detected by the instantaneous oxygenutilization, or the rate of change therein, in a manner that isotherwise substantially similar to detection by vent gas O₂ per se.

Oxygen utilization is preferably determined by comparing the oxygen flowrate to the reactor with the product of the vent gas flow rate and thevent gas O₂ content as determined in the gas phase exiting the liquidphase. For the latter determination, a segregated sample is preferablyused to produce a segregated vent gas sample that is analyzed for O₂.Alternatively, a probe immersed in the liquid phase can be used asdescribed above.

Similarly, as further demonstrated in FIG. 20, the CO₂ content of thevent gas falls sharply in the last few minutes before the typical endpoint. Thus, as an alternative to using the data reflected by the CO₂vent gas profile in the aforesaid cumulative and instantaneous CO₂generation methods (or in combination therewith), the instantaneous CO₂content of the vent gas, and/or the rate of change thereof, may itselfprovide a useful end point indication for the reaction system.Historical analytical data may be used in the same manner to calibrateend point detection by vent gas CO₂ content as described above for ventgas O₂ content. Application of CO₂ content end point detection to batch,continuous back mixed and continuous plug flow systems is also the sameas for vent gas O₂ content as described above, including the methodsdescribed for isolating a sample of the vent gas evolving from theliquid phase or use of a probe immersed in the aqueous liquid. Furtherrefinement of end point may be realized by following both vent gas O₂content and vent gas CO₂ content. Either may be used to cross checkand/or adjust the other.

As further illustrated in FIG. 18, dissolved oxygen content of theaqueous reaction medium may also be used in the manner immediatelydescribed above for vent gas O₂ and/or CO₂ content in identifying endpoints at which residual PMIDA concentration has been reduced to adesired level. In the particular operations illustrated, a sharperresponse may be observed in the vent gas O₂ and CO₂ content, as comparedto dissolved oxygen. As noted, oxygen flow rate is typically reducedduring the final minutes of reaction to minimize AMPA formation andavoid oxygen poisoning of a noble metal catalyst, and this can damp theeffect of declining reaction rate on dissolved oxygen. However,depending on the selection of catalyst and other conditions imposed onthe process, a reasonably sharp end point identification may be obtainedby monitoring this variable. Like the other methods, end point detectionby dissolved oxygen level may be calibrated by comparison to historicalFTIR or HPLC analytical data obtained in a laboratory or industrialoxidation reactor.

In accordance with a further alternative method of the invention, theconversion of PMIDA can be monitored and/or an end point defined by atarget residual PMIDA concentration can be projected based on in-lineFTIR or HPLC analysis for one or more products of over-oxidation suchas, for example, aminomethylphosphonic acid (“AMPA”). Based on synthesisof historical analytical data obtained from operation of a laboratory orindustrial scale oxidation reactor, a fundamental or empiricalcorrelation can be developed between AMPA accumulation and conversion.Such correlation may vary depending on whether the reaction systemcomprises a batch reactor, a series of continuous back mixed reactors,or a plug flow reactor. Based on the same or similar data, a correlationmay also be developed between AMPA accumulation and reaction time for abatch reactor, especially toward the end of the reaction cycle; or AMPAaccumulation as function of residence time in the last of a series for acontinuous back mixed reactors or at the exit of a plug flow reactor.Combining a correlation of PMIDA content vs. AMPA content with acorrelation of AMPA content vs. time, an end point projection algorithmmay be developed based on measured AMPA content. If AMPA response issharper than PMIDA response, the precision of end point detection may beenhanced vs. a method based on direct analysis for PMIDA alone.Conversion and end point projections based on accumulation of AMPA, orother over-oxidation products, can be compared with data from othermethods for conversion monitoring as discussed herein, and thecomparison integrated into a general program for estimating and crosschecking such projections.

According to a further alternative, the PMIDA conversion and/or reactionend point can be monitored or determined by the electrochemical responseof the aqueous reaction medium to an imposed current or imposed voltage.In such methods, a potential is imposed between a working electrode andcounterelectrode, both electrodes being immersed in the aqueous reactionmedium or a sample thereof. An estimate of the extent of conversion andof residual PMIDA content can be determined from a function of the powerthat is consumed in maintaining a select current density or selectpotential difference between the electrodes. The methods are based onthe difference in the potential required for the electrochemicaloxidation of PMIDA, which is relatively low, vs. the potential requiredfor oxidation of glyphosate, which is relatively high. At low tomoderate conversions, the current flows between the electrodes at avoltage effective for the electrochemical oxidation of PMIDA, butinsufficient for the electrochemical oxidation of glyphosate. When PMIDAis sufficiently depleted, the current flow either sharply declines, oris substantially shifted to electrochemical oxidation of glyphosate. Ineither case, the current/voltage relationship changes so that greaterpower consumption is required to maintain a given current density.

In one alternative for electrochemical detection or monitoring of theconversion of PMIDA, a select current density is maintained between theelectrodes, and the voltage required to maintain that current density iscontinuously or intermittently measured. The select current densitymaintained between the electrodes immersed in the aqueous reactionmedium or a sample thereof is preferably held substantially constant,but optionally may be a programmed current density, e.g., a series ofdiscrete current densities or a current scan. Extinction of PMIDA to atarget residual level is detected by a rise in voltage necessary tomaintain the select current density. The voltage that is necessary tomaintain the select current density at the target PMIDA content isessentially the potential required for oxidation of PMIDA, plus anincrement necessary to overcome the resistivity of the solution betweenthe electrodes and any fouling or other source of polarization at theelectrodes. Because the C₁ by-product compounds formaldehyde and formicacid are subject to electrochemical oxidation at a potential lower thanthat required for oxidation of PMIDA, the imposed or select currentdensity is the sum of that sufficient for oxidation of the C₁s andpossibly other readily oxidizable impurities, plus an increment thatprovides the Faradaic equivalent of a target PMIDA concentration. Duringoperation at high to modest PMIDA concentration, the current density iscarried entirely by the oxidation of C₁s, other readily oxidizableimpurities and PMIDA; and the voltage stays relatively constant at alevel slightly above the potential required for PMIDA oxidation. As thePMIDA concentration drops below the target threshold, the sum of theoxidation products of C₁, other readily oxidizable impurities and PMIDAis no longer sufficient to carry the select current density, and thevoltage increases to the potential required for oxidation of glyphosate.In a batch reaction, this end point may be identified by a sharpinflection in a plot of voltage vs. time, e.g., as presented on aprocess operations control chart.

As further discussed herein, “select current density” and “currentdensity” are sometimes referred to as “select current” and “current,”respectively. At a voltage sufficient for electrooxidation of PMIDA butnot glyphosate, the sum of the PMIDA and C₁ content is a function ofcurrent density rather than absolute current, because the currentdensity is a function of the absolute current and the scale and geometryof the electrolytic circuit, it depends on the area and orientation ofthe electrodes presented to the solution in the electrolytic circuit.Those skilled in the art will recognize that, in the practicalapplication of the method of the invention in a manufacturing facility,once the structure of the circuit and circuit elements, includingelectrodes, is fixed, the variable actually imposed and controlled, andagainst which the requisite voltage is measured and displayed, may bethe current rather than current density. But for the same reasons, oncethe scale, structure and geometry of the system is fixed, selection ofcurrent is tantamount to selection of current density.

A plot of voltage vs. time (and conversion) at constant current densityis illustrated in FIG. 7. This is illustrative of the readout obtainedfrom a process instrument used for monitoring PMIDA conversion and/orestimating or identifying a desired end point. FIG. 8 comprises a seriesof voltage vs. time and conversion plots of the type that may be used inselection of programmed current that is most effective for detecting thedesired end point/degree of conversion. In FIG. 8, voltage is recordedas a function of time at a variety of different select currentdensities. It will be seen that if the select current density is toohigh, solution impedance obscures the effect of electrochemicaloxidation, and a sharp inflection is not observed when the PMIDAconcentration drops to a target value. A lower current, on the otherhand, is potentially effective for identifying a very low targetresidual PMIDA content, and thus a high conversion. However, if thecurrent is too low, the end point may be obscured by the backgroundcurrent or be carried entirely by oxidation of formaldehyde, formic acidand various readily oxidizable impurities. Thus, if too low a current isused in end point detection, it may result in the catalytic oxidationreaction cycle being prolonged to the extent that a risk of overreactionis incurred, i.e., oxidation of product glyphosate to AMPA. In the plotin question, the optimal current is between about 4 and about 7milliamps.

More generally, it is preferred that the select current density be inthe range between about 0.1 and about 0.7 mA/mm², preferably betweenabout 0.2 and about 0.5 mA/mm² at the working electrode, i.e., theanode. Thus, for example, where the electrodes comprise parallel pins ofabout 1.5 mm diameter×3-5 mm in length, the preferred current maytypically fall in the 4 to 7 mA range indicated as optimal in FIG. 8. Inorder to minimize the effect of solution resistivity, the electrodes arepreferably spaced between about 1 mm and about 4 mm apart, preferablyabout 2 to about 3 mm apart. Advantageously, the flow rate of reactionsolution between the electrodes is maintained at a least about 100cm/sec, more typically between about 30 and about 300 cm/sec to minimizefouling and polarization and to maintain a representative solutionaround the electrodes.

Subject to the sensitivity limitations indicated in FIG. 8, tovariability in the C₁ concentration at the end of the batch, and to thepredictability of C₁ variations, the select current method may be tunedto identify essentially any target end point for a batch reaction. Forexample, a constant current electrochemical detection system may be setto identify an end point of 450 to 600 ppm PMIDA for standard operationsor in the range of 45 to 60 ppm where an exceptionally low PMIDA productis desired. Where the process comprises recycle of the PMIDA from theglyphosate product recovery area to the reaction system, the target endpoint may be increased by an increment corresponding to the extent ofthe recycle. For example in a continuous process of the type illustratedin FIGS. 1 and 2, the target end point may typically be from about 500to about 2500 ppm PMIDA for standard operations, or in the range ofabout 250 ppm PMIDA under conditions where an exceptionally low PMIDAproduct is desired.

In a continuous process, select current may be established in a streamof sample of the product reaction solution in or exiting the finalreactor. If the voltage required to maintain the imposed currentapproaches or exceeds the potential required for electrochemicaloxidation of glyphosate, the target PMIDA has been reached; if not,there may still be too much unreacted PMIDA. A more specificdetermination of actual PMIDA content can be obtained by scanning thecurrent and observing the voltage, subtracting the componentattributable to C₁s as achieved at voltages below the thresholdpotential for electrochemical oxidation of PMIDA, and estimatingresidual PMIDA from the Faraday equivalent of the current increment inexcess of that required for C₁s.

To minimize effects of fouling and polarization, the polarity of theelectrodes is repetitively reversed, so that what had been the workingelectrode becomes the counterelectrode and vice versa. Reversal ispreferably effected at intervals of not more than about 10 minutes, moretypically not more than about one minute, preferably not greater thanabout 30 seconds, most typically between about one second and about 20seconds, preferably between about 5 seconds and about 15 seconds. Abipolar power source is provided for this purpose. FIG. 17 illustratesthe typical voltage vs. time chart that is recorded using a selectcurrent electrochemical oxidation detection method with polarityreversal at 2.7 second intervals. The dashed trace is recorded early inthe batch and shows that no increase in voltage is required to maintaina select current density, indicating that the current is all beingcarried by oxidation of C₁s and PMIDA, with no participation byglyphosate. The solid trace is taken later in the batch and shows thateach of the alternating current pulses requires a ramp in voltage,reflecting the requirement for oxidation of glyphosate in order to carrythe full select current density.

FIG. 18, which is discussed above with respect to oxygen flow anddissolved oxygen profiles, also comprises a trace of the voltageresponse observed in practice of a select current electrochemicaloxidation end point detection method. It may be seen that the voltagedeclines as the aqueous reaction medium is brought to reactiontemperature, goes through a trough early in the batch and then climbsalong a modest slope until shortly before the end point is approached.In the last few minutes of the reaction cycle, the voltage responseclimbs relatively steeply (typically though not necessarilyexponentially) to a level which indicates sufficient conversion for theoxygen flow to be terminated. Upon termination of oxygen supply, thedissolved oxygen content drops precipitously to essentially zero, andthe voltage increases nearly instantaneously to the level at whichelectrochemical oxidation of PMIDA can occur in the absence of dissolvedoxygen. As discussed elsewhere herein, purely electrochemical oxidation,unaided by dissolved oxygen, typically requires a potential in excess of3 volts, more typically more than 3.5 volts.

According to an alternative method for electrochemical detection ormonitoring of the conversion of PMIDA, a select voltage may be appliedwhich is sufficient to effect electrochemical oxidation of PMIDA, butnot glyphosate, and measurement made of the current response to theapplied voltage. The result may be adjusted for C₁s, which are alsooxidized at a voltage effective for the oxidation of PMIDA. The C₁content at high PMIDA conversion can be estimated based on long term orshort term historical analytical data, as described above with respectto the oxygen consumption and carbon dioxide generation methods, and thecurrent equivalent thereto estimated according to Faraday's Law. After acurrent component equivalent to the C₁s is subtracted, the remainingcurrent increment is substantially proportional to the residual PMIDAcontent of the aqueous reaction medium. Reduction of the residual PMIDAcontent to the target value is indicated by a decline in the currentresponse as compared to that obtained at a higher PMIDA concentration.

According to a refined alternative, the select voltage method allows theC₁ content to be repetitively determined, and thus a zero value for thePMIDA determination to be provided, by applying two discrete voltages insequence. The first, relatively lower voltage, is effective for theelectrochemical oxidation of formaldehyde and formic acid, but not forthe electrochemical oxidation of PMIDA. The second, relatively highervoltage, is sufficient for the electrochemical oxidation of PMIDA butnot for the oxidation of glyphosate. The current response to the firstapplied voltage reflects oxidation only of the C₁s, while the currentresponse to the second applied voltage reflects the oxidation of boththe C₁s and PMIDA. The PMIDA content is indicated by the differencebetween the two current responses.

The various species within the oxidation reaction mixture, includingformaldehyde, formic acid, PMIDA and glyphosate are all subject toentirely electrolytic oxidation in aqueous media at voltages that arerelatively high. For example, it is known that glyphosate may beproduced by the electrolytic oxidation of PMIDA at potentials in therange of 3.3 volts or greater. However, in the methods of the invention,PMIDA conversions are typically estimated in a reaction medium throughwhich molecular oxygen is being constantly sparged. Thus, the medium hasa dissolved oxygen content, and consequently an oxidation potential,that are substantial fractions of the values prevailing at oxygensaturation. In the presence of an adequate supply of molecular oxygen,the various electrolytic reactions proceed via the reduction ofmolecular oxygen at net voltages that are substantially lower than thoserequired for electrolytic oxidation alone.

For example, oxidation of PMIDA to glyphosate proceeds typically at avoltage of about 0.7 or greater relative to a Ag/AgCl electrode. Bycomparison, oxidation of formaldehyde and formic acid to carbon dioxideand water proceeds at a voltage in the range of about 0.4 relative to aAg/AgCl electrode, while oxidation of PMIDA to glyphosate requires avoltage in the range of 1.2 to 1.3.

In operation of the select voltage method, pulses are alternately andrepetitively applied at a plurality of different voltages. One voltage,a relatively lower voltage, is sufficient for oxidation of C₁ compoundsbut not PMIDA. Another voltage, a relatively higher voltage, issufficient for oxidation of both C₁s and PMIDA. The current or currentdensity responses and difference between such responses are determinedfor each successive combination of low voltage and high voltage pulsepairs. The current difference is continually tracked. In a batchreaction, the end point is reflected by a sharp drop in the currentobtained at the higher voltage, and more particularly by a sharp drop inthe difference between high voltage and low voltage current as computedfrom pulse cycle to pulse cycle.

The principles and mode of operation of the select voltage method areshown in FIG. 9. At periodic intervals, a first voltage V₁ is appliedwhich is sufficient for the electrochemical oxidation of by-product C₁compounds, i.e., formaldehyde and formic acid, but not sufficient forthe electrochemical oxidation of either PMIDA or glyphosate. The currentI₁ generated in response to V₁ reflects only the concentration of theaforesaid C₁ compounds (and possibly other minor backgroundcontaminants) and not the concentration of either PMIDA or glyphosate.At a different point in the select voltage method cycle, a highervoltage V₂ is applied which is sufficient to oxidize the C₁ compounds(and any background contaminants) and PMIDA, but not sufficient tooxidize glyphosate. The current response to V₂ is I₁₊₂. The differencebetween the two observed current responses is I₂, the current that isattributable to the oxidation of PMIDA. As the reaction progresses, I₁may continually increase, or may increase to a maximum and then remainlevel or decline; but a significant residual C₁ content typicallyremains in the reaction mixture even as the PMIDA end point is reached,especially in the case where the catalyst comprises carbon only.Regardless of the profile of I₁ across the reaction, I₂ declinesprogressively as the reaction proceeds until the target PMIDAconcentration is reached.

In implementation of both the select current and select voltage methods,the electrodes may be constructed of any convenient material that has alow oxidation potential and is chemically inert in the system. As apractical matter, these considerations tend to narrow the options.Platinum is preferred for its chemical and electrochemical inertness. APt/Ir electrode combines inertness with mechanical strength. In variouspreferred embodiments of the select voltage method, as illustrated inFIG. 19, a reference electrode, e.g., a pseudo Ag/AgCl electrode isprovided in proximity to the working electrode (anode) at which theoxidation reaction takes place. Because no current flows at thereference electrode, it functions reliably to sense the voltageprevailing at the working electrode so as to assist in controlling thelatter at a target voltage such as V₁, for oxidation only of C₁compounds to generate current I₁, or V₂, for oxidation of both C₁compounds and PMIDA without oxidation of glyphosate to generate currentI₁₊₂. By sensing the voltage at the working electrode, the referenceelectrode provides the basis for controlling that voltage value at adesired level. In the absence of the reference electrode, control of theworking electrode at a proper voltage for the desired electrochemicaloxidation reaction may be compromised by the resistivity of the solutionand other vagaries of the environment in which the electrolytic circuitmust function.

Alternatively, the reference electrode may be used merely to sense thevoltage at the working electrode, without being part of any controlcircuit for maintaining that voltage at a target level. Instead, asfurther discussed hereinbelow, the voltage as sensed may be used as aterm in a regression equation for computing PMIDA content from this andother measured parameters of the reaction system. FIG. 19 illustratessuch a system in which a voltage is applied from a power source 701across a working electrode 703 and a counterelectrode (auxiliaryelectrode) 705 that are immersed in the process stream, and theresulting current measured by means of an ammeter 707. Via apotentiometer 711, the voltage across reference electrode 709 andworking electrode 703 is sensed.

In some instances, it may be desirable for the reference electrode to bebased on a redox couple of known oxidation potential, e.g., Ag/AgCl.

However, in some applications, especially where the aqueous mediumflowing past the electrode contains a particulate catalyst, a Agelectrode may rapidly erode or corrode. In such applications, it may bepreferable for the reference electrode to also be formed of Pt or aPt/Ir alloy. In the latter instance, the reference electrode may nothave a known potential, but since no current flows at this electrode itfacilitates the means by which the working electrode voltage may besensed and controlled.

Even where a reference electrode is used to assist in measurement of theworking electrode voltage at a value effective for the desiredelectrochemical reactions, variables other than the power consumptionresponse to the select current or select voltage have been found toaffect the precision of the electrochemical methods for PMIDAconversion. These include for example, the absolute voltages (ascontrasted with the voltage difference), the rest potential betweenpulses, i.e., the oxidation potential of the medium, and thetemperature. Rest potential may be determined with an ORP electrode, asdiscussed hereinabove. In the select voltage method, the effect of suchother variables may be taken into account using an algorithm that may bedeveloped for the purpose. Advantageously, compensation for thesevarious effects may be accounted for on-line by transmitting signals forcurrent, current difference, temperature, absolute voltage and restpotential to a processor which is programmed with the algorithm.

Although the electrochemical methods for estimating PMIDA concentrationoperate on known principles, it has been found that the vagaries of anoperating environment are not always susceptible to evaluation by purelyscientific calculation, especially at relatively low PMIDAconcentration. Instead it has been found that estimation of the PMIDAcontent may be best achieved by development and application of anempirical algorithm that is generated by regression analysis ofextensive operating data. Thus, a typical empirical relationship is asset forth below:

PMIDA(ppm)=C ₁(P1OV)+C ₂(P1OC)+C ₃(P2OV)+C ₄(P2OC)+C ₅(RV)+C ₆(PT)+C₇  (Eq. 1-1)

Where:

P1OV=pulse 1 observed voltage

P1OC=pulse 1 observed current

P2OV=pulse 2 observed voltage

P2OC=pulse 2 observed current

RV=rest voltage

PT=process temperature

C₁, C₂, C₃, C₄, C₅=regression equation coefficients

C₇=a constant

The values of the coefficients may vary significantly between batch vs.continuous mode oxidation, filtered vs. unfiltered aqueous medium, typeand age of catalyst, and numerous other variables of the process,equipment and control system in the manufacturing facility in which themethod is used. Moreover, because the coefficients are empirical asdetermined by regression analysis, they are typically sensitive tomodest variations in system parameters, and can shift by orders ofmagnitude. In such instances, an order of magnitude change in onecoefficient may be offset by an order of magnitude change in othercoefficients of opposite sign. Typically, it may also be necessary toapply an offset or correction factor to the value as calculated by anequation of the nature set forth above. It will be understood thatduring any particular period of operation, the range of the aforesaidcoefficients and correction factor may vary significantly. In thisregard, an empirical regression equation for residual PMIDA content isgenerally valid over a PMIDA range of no more than about two orders ofmagnitude. For example, separate algorithms may be necessary forcomputation in a range of 200 to 2000 ppm vs. a range of 2000 to 8000ppm. It has been found that a logarithmic model may reconcile data fromthese separate ranges, but more accurate operating information isprovided by applying separate linear equations to separate ranges ofPMIDA content.

Electrodes for electrochemical estimation of conversion are preferablypositioned in a location where temperature is substantially constant andeither equal to or consistently reflective of the bulk temperature inthe catalytic oxidation reaction zone. It may also be advantageous toposition the electrodes in an area of relatively high flow in order tominimize fouling and polarization of the electrodes. In the selectcurrent method, it is also preferable to maintain the flow rate andother measurement variables as nearly constant as is practicable,including oxygen potential of the solution, solution conductivity, andelectrode disposition and dimensions. Where the reaction zone comprisesa stirred tank reactor having an external heat exchanger for removingthe exothermic heat of reaction, the electrodes are advantageouslypositioned in the circulating line immediately upstream of the heatexchanger; or in a slip stream parallel to the main circulating stream,but also preferably just upstream of the heat exchanger. This positionaffords both constant temperature (bulk temperature of the aqueousmedium) and flow.

In certain applications, it may be desirable to periodically reverse thepolarity of a select voltage electrochemical oxidation circuit. Forexample, in a continuous process comprising CSTRs in series, remainingPMIDA content may advantageously be estimated for a stream exiting aCSTR other than the final reactor in the series, e.g., either thepenultimate reactor or the third last reactor. In such instance, theresidual PMIDA content may be high enough to cause polarization byaccumulation of a high concentration of glyphosate in a boundary layeralong the surface of the electrode and/or fouling of the electrode bydeposit of solid glyphosate. The solubility of glyphosate in the aqueousmedium is limited. If glyphosate accumulates to a high enoughconcentration in the boundary layer, it can precipitate on the electrodesurface. This may be manifested by drift in the response at differentvoltages and residual current when the circuit is otherwise at rest.Periodic reversal of polarity helps to prevent concentrationpolarization and fouling. The frequency of reversal depends on theenvironment in which the select voltage system is applied. Typically,reversal may be effected every 15 seconds to several minutes, moretypically between about 15 seconds and about two minutes, still moretypically between about 20 seconds and about one minute.

The select voltage method is somewhat more adaptable to changingconditions, in part because a zero basis is established by measuringboth the current attributable to oxidation of C₁s only and the currentattributable to oxidation of both C₁s and PMIDA, and further because ofthe availability of an algorithm that compensates for changes intemperature, absolute voltage and rest potential.

FIG. 10 schematically illustrates a control system for implementing theselect voltage method for tracking the conversion of PMIDA. A computercontroller 201 is programmed to impose a select voltage between workingelectrode E₁ and counterelectrode E₂. In response to a voltage signalfrom across reference electrode E₃ and working electrode E₁, an outputsignal from the computer adjusts impedance in the electronic circuitry203 to maintain the voltage at the desired value V₁ or V₂ as the casemay be, depending on which cycle the computer is running. The resultingcurrent (I₁ or I₁₊₂) is sensed by a current sensor in the circuitry 203and a current signal transmitted to the computer 201. Voltage V₁, whichis sufficient only for C₁ compound oxidation, is applied alternately toa voltage V₂, which sufficient for oxidation of both C₁ compounds andPMIDA. Typically, each voltage is applied for a period of between about10 and about 20 seconds, with a rest interval between applications oftypically between about 10 and 20 seconds. The resulting current I₁ orI₁₊₂ is sensed across the working electrode E₁ and the counterelectrodeelectrode E₂ and transmitted to the computer processor 201. The computeris programmed with an algorithm by which the current I₂ can be computedfrom measured currents I₁ and I₁₊₂, and from which the computerdetermines the residual concentration of PMIDA, effectively from thedifference between I₁ and I₁₊₂, i.e., I₂, but preferably frommeasurements that include applied voltages and rest potentials, forexample per regression equation 1-1. An output signal is transmitted toa readout device that may be positioned on a control panel and/or on orin the vicinity of the reactor. As further discussed hereinbelow, theoutput signal from computer 201 may be used in feedback control of anindependent process variable such as, e.g., the reaction temperature,intensity of agitation, the rate and/or pressure of oxygen supply or, ina continuous reaction system, the rate of introduction of PMIDA feedsolution to the reactor and the rate of withdrawal of product reactionmixture therefrom.

Select current and select voltage electrochemical oxidation methods canalso be used in combination to estimate conversion and/or determine endpoint of the oxidation reaction. One alternative which can be used forthis purpose, and interpreted with respect to either voltage as functionof current or current as a function of voltage, is the current scanningmethod that is described hereinabove. Whether current response tovoltage and voltage response to current are gathered dynamically byscanning, or by separately applied select current and select discretevoltage(s), an algorithm may be derived by regression analysis usingcurrent response to applied voltage and voltage response to appliedcurrent density. The product of each of these responses and anappropriate coefficient is incorporated as a term in a polynomialexpression for residual PMIDA content of the reaction medium. Generally,voltage response to select current appears as a negative term in anexpression for residual PMIDA, while current response to select voltageis a positive term. The precision of the determination can be enhancedby including the rest potential, which essentially corresponds to theoxidation potential of the aqueous medium, and can be measured using anoxidation/reduction potential probe. Generally, the oxidation potentialappears as a negative term in the regression equation. The coefficientsare specific to the conditions of the particular reaction, the nature,loading, age and activity of the catalyst, and at least potentially tothe peculiarities of the particular reactor configuration. However,based on the description herein, they can be readily derived by thoseskilled in the art from standard regression analysis.

In practice of the select voltage method, or in sweeping the voltage,data may be gathered by which the requisite batch reaction time orcontinuous reactor residence time necessary to obtain a targetconversion and/or residual PMIDA content may be projected in the mannerdescribed above in the case of FTIR. For example, a series of currentresponses may be obtained in response to a select applied voltage, or aplurality of discrete select voltages. Based on a known or determinedorder of reaction, these responses may be used to project the batchreaction time or continuous reaction residence time necessary to achievea target conversion of PMIDA to glyphosate or another intermediate forglyphosate and/or a target end point defined by residual PMIDAconcentration. The projection may be made based on a known or determinedrelationship between residual PMIDA content and the current response ata plurality of the series of applied voltages. At least two or more ofthe applied current determinations are preferably obtained undernon-zero order reaction conditions. As in the case of projections basedon FTIR, oxygen consumption, or CO₂ generation, the order of thereaction and the rate constant may be determined from historical HPLCdata, FTIR data, other analytical data, or operational data obtainedfrom laboratory and/or industrial oxidation reactions. Where theprojection is made on the basis of substantially first order reaction,the end point may be projected based on a straight line plot of thelogarithm of the remaining PMIDA concentration vs. time.

Where the electrodes used in either the select current or select voltageelectrochemical oxidation method are inserted in a recirculation line,e.g., a slip stream of the reaction mixture circulating through anexternal heat exchanger, the electrodes may conveniently be mounted on aprobe that is inserted in a pipe spool such as that illustrated in FIG.13. A flanged pipe pool 301 is provided with an internally threadedlateral entry coupling 303 that is adapted for insertion of the probe.As illustrated in the drawing, the spool includes a second internallythreaded coupling 305 so that two probes can be accommodated by the samespool. The probe as illustrated in FIGS. 11 and 12 includes a bushing307 that is threadably received in coupling 303 or 305, and a cable 309which carries leads for the electrodes. The probe is sealed againstleakage of process liquid by an O-ring 313. The cable passes through andis sealed within the bushing. As shown in FIG. 12, the probe comprisesthree electrodes, i.e., working electrode E₁, counterelectrode E₂, andreference electrode E₃. FIG. 16 is an end view of a two electrode probe.Thus, the electrode probe may be adapted for use in either the selectcurrent or stepped voltage method as described above. Typically,coupling 305, bushing 307 and electrodes E₁, E₂, and E₃ are positionednormal to the direction of flow of aqueous reaction medium through spool301. An annular shroud 311, generally coaxial with bushing 307 andcoupling 305, serves to provide at least some reasonable degree ofprotection for the electrodes against erosion that otherwise may resultfrom action of the flowing reaction mixture, and especially catalystthat is ordinarily suspended therein. The shroud may be slotted atpositions around its circumference to allow adequate circulation of bulkliquid to the electrode surfaces. The shroud and other wetted surfacesof probe are preferably constructed of a corrosion resistant alloy suchas, for example, alloy 825 of alloy 276.

In providing for two separate probes, the arrangement of FIG. 13 allowsfor operation using a variety of different electrode combinations; andin particular for redundant measurement against the contingency that oneelectrode probe may become fouled or polarized and give a false reading.Thus, for example, using the arrangement provided in FIG. 13 theelectrochemical detection methods may use an electrochemical circuitcomprising a single dual electrode probe of the type illustrated inFIGS. 11 and 12, or two single isolated electrodes; and may provide forredundant measurement, or two dual isolated electrodes.

For proper calibration and operation of the select current and selectvoltage electrochemical oxidation methods, it is desirable to establishwhether: (i) the aqueous medium subject to the method has an oxidationpotential sufficient for the various reactions to proceed in the voltageranges described above; or instead (ii) the oxidation potential is solow that detection can proceed only at the relatively high potentialsthat are necessary for solely electrolytic oxidation. Ordinarily, it mayalso be desirable to maintain the oxidation potential of the analytemedium in one condition or the other, i.e., at an oxidation potentialsufficient for PMIDA to be electrochemically oxidized in the range of0.7 to 1.0 volt, or at oxidation potential insufficient for oxidation ofPMIDA to proceed until a significantly higher threshold voltages isimposed.

Measurement of oxidation voltages and currents may further becomplicated by the presence of catalyst in the aqueous reaction mediumthat is subjected to the end point detection method. It has beenobserved that the presence of catalyst tends to promote higher currentsat a given voltage and vice versa. Because the catalyst has asignificant oxygen carrying capacity, it is believed that the effect ofcatalyst may be to transport oxygen to the electrodes, either bycollisions between catalyst and the electrodes or by oxygen enrichmentof the solution in proximity to the electrodes. With reference to FIGS.1 and 2, for example, the voltage and current responses to an electrodeprobe placed in one or more of reactors 101, 103 and 105, or in acirculating line for an external heat exchanger associated with suchreactor(s), may differ significantly from the response obtained byapplication of current or voltage to a sample of the aqueous medium fromwhich the catalyst has been removed, as by filtration. Similarly, theresponses obtained from a probe in a reactor or reactor circulating linemay differ significantly from the response that is observed uponapplication or current or voltage to the aqueous reaction mediumdownstream of filter 107. The select voltage method may be calibrated byperiodic assays of the reaction mixture and comparison of current atdiscrete select voltages with assay for PMIDA, formaldehyde and formicacid.

FIG. 14 illustrates a scheme for control of the PMIDA conversion in areaction system of the type illustrated in FIGS. 1 and 2, and moreparticularly for controlling the conditions of the reaction to achieveand maintain a target PMIDA concentration in the reaction medium exitingthe reaction system. In the process control scheme of FIG. 14,conversion of PMIDA is established and maintained by an appropriatecombination of reactor residence time, PMIDA concentration, reactiontemperature, intensity of agitation and oxygen flow rate. A conventionalprocess control instrumentation system, indicated collectively andschematically at 403, is provided for measurement and control of PMIDAfeed rate, temperature in the reactors, and calculation of oxygen flowsetpoints to each of reactors 101, 103 and 105. Signals from sensors andcontrollers including but not limited to those listed in block 415, aretransmitted to 403 for adjusting the control set points of oxygen intothe several reactors 413.

A select voltage or select current system 405 provides a raw estimate ofresidual PMIDA content based on the response obtained in an appropriatereaction stream or sample. For example, an electrochemical detectionprobe may be positioned in reactor 105, in a line circulating aqueousreaction medium between reactor 105 and an external heat exchangerassociated therewith, or in the line exiting filter 107. Optionally,measurements may be taken in both reactor 105, or its circulating line,and in the exit stream from filter 107. The measurement in reactor 105or its circulating line provides a value that may be used for controlwith somewhat less response lag than is incurred where the controlfunction responds to the PMIDA value obtained in the stream exitingfilter 107. Otherwise, placing a probe in the filtrate is generallyadvantageous because it measures PMIDA content in the substantialabsence of catalyst, and is, therefore, less subject to vagaries ofcatalyst activity, concentration, etc. However, the stream exiting thefilter may in some instances have a relatively low dissolved oxygencontent. Care must be taken to determine whether the oxidation potentialof the filtrate is sufficient for electrochemical oxidation in thepreferred ranges discussed above. Optionally, the filtrate may beaerated to make certain that its oxidation potential is sufficient foroxidation of PMIDA at a voltage in the range of about 0.7 to about 1.0.Alternatively, the filtrate may be purposefully subjected toelectrochemical oxidation under oxygen starved conditions, in which caseC₁ oxidation may typically be observed in the range between about 1.5and about 2 volts, and PMIDA oxidation may be observed in the rangebetween about 3.5 and about 5 volts.

A raw estimate of PMIDA concentration is computed by function block 405according to an algorithm such as equation 1-1 above. A correctionfactor for the raw PMIDA estimate is computed in function block 409according to an empirical correction algorithm which compares previousraw PMIDA estimated from 405 with measured laboratory values at knownPMIDA concentration. A function block 407 applies the correction factorto a raw PMIDA computation to obtain a corrected PMIDA concentration.Signals reflecting the corrected PMIDA concentration are transmitted asan input to process control system 403, which is programmed with analgorithm for computing adjusted set points of the oxygen flow 413 basedon inputs from O₂ flowrates 411 at levels calculated to afford thedesired conversion and residual PMIDA content in the aqueous productreaction mixture exiting reactor 105 and/or filter 107 as specified bymanual inputs from 401.

In some instances, it may be useful to not only compensate for theoxidation of C₁s in estimating residual PMIDA content by electrochemicaloxidation, but also to obtain a separate estimate of the residualconcentration(s) of the C₁ compounds themselves. The principles asapplied above may be applied to obtain such indication. In the selectvoltage method, for example, a separate algorithm may be developed byregression analysis comparing I₁ with analytical data for formaldehydeand formic acid, and with other parameters of the process such astemperature, rest potential, etc.

As described in further detail above with respect to FTIR, and belowwith respect to heat generation, in a batch mode the rate constant andthe order of the reaction may be estimated from the rate of decline inthe reaction rate as a function of time as determined from a pluralityof analyses and/or current responses and/or other operational dataobtained during the course of the reaction in which the end point isprojected, or from a preceding batch. More particularly, the rateconstant may be estimated from operational data based on the rate ofexothermic heat generation, the rate of oxygen consumption in thereaction zone, the rate of generation of CO₂ in the reaction zone, orcombinations thereof.

Similar determinations can be made from data obtained at differentresidence times in a continuous reaction system. For example, where thereaction is conducted in a continuous back mixed reaction zone undernon-zero order conditions, a projection may be made based on the PMIDAcontent of the reaction medium exiting the penultimate reactor. Theorder of the reaction and the kinetic rate constant can be estimatedbased on historical operational or analytical data obtained fromlaboratory and/or industrial oxidation reactions, including recentlypreceding operations in the continuous back mixed reaction zone. Forexample, the kinetic rate constant may be estimated from the PMIDAcontent of the feed solution entering the back mixed reaction zone, andthe PMIDA content of the solution withdrawn from the back mixed reactionzone as a function of the residence time therein.

According to a still further alternative, the PMIDA conversion and/orend point may be estimated from cumulative heat generation in thereaction zone during the course of the reaction. Because the oxidationreaction is exothermic, means are provided for transfer of reaction heatfrom the reaction mixture under feedback temperature control. Thus, if acooling fluid such as a source of cooling water is passed through a heatexchanger, e.g., cooling coils, a cooling jacket or an external heatexchanger, for controlling the reaction temperature, the extent ofreaction can be estimated from the cumulative heat dissipation over thebatch cycle, as may be determined from an integrated average of theproduct of cooling fluid flow rate and temperature rise through the heatexchanger during the course of the batch. Thus, the method comprisescontinually or repetitively measuring heat generation during the courseof the reaction, preferably by continually or repetitively measuringboth the flow rate of the coolant and the temperature rise through theheat exchanger, and computing the cumulative heat generation within abatch reaction zone at any point in a batch reaction cycle, or in acontinuous reaction zone comprising a particular reactor or the total ofall reactors in a series of CSTRs. To estimate the conversion, the heatgenerated in the reaction zone is compared with the mass of PMIDAcharged to the reaction zone and the exothermic heat of reaction for theoxidation of PMIDA to glyphosate. In a continuous reaction system, suchcomparison can be made over a select time period, or repetitively oversimilar select time periods. Based on historical analytical data, thevalue thus obtained can be adjusted for heat generated in the oxidationof formaldehyde to formic acid, and formic acid to CO₂. Alternatively,an estimate of residual C₁ content may be based, for example, on FTIR orelectrochemical oxidation data. Using residual C₁s concentrations asdetermined by electrochemical oxidation, a material balance may becomputed to determine the quantities of formaldehyde and formic acidconsumed by oxidation vs. vent loss. A C₁ heat balance based on thematerial balance provides a C₁ oxidation heat component by which thegross heat generation observed can be adjusted to determine the quantityof heat associated with oxidation of PMIDA. For this purpose, the PMIDAcharge may be either the initial charge to a batch reaction zone or thequantity charged to a continuous reaction zone over the period duringwhich the cumulative heat generation is measured. In estimatingconversion from the cumulative heat generation, the precision of theestimate is enhanced by a careful and accurate measurement of the PMIDAcharge to a batch reactor, or of the integrated average instantaneousrate at which PMIDA is charged to a continuous reaction system.

Practice of the cumulative heat balance method for estimation of PMIDAconversion is illustrated in FIG. 15. As schematically illustrated, thereaction system comprises a single batch oxidation reactor 501 and asingle external heat exchanger 503 through which the reaction mixture iscirculated during the course of reaction. A cooling fluid is passedthrough exchanger 503 at a rate which is regulated by a control valve505 and by a temperature controller 507 in response to a reactionmixture temperature sensor 509 to maintain a constant temperature of thereaction mixture. Temperature sensors 511 and 513 continually measurethe temperature of the cooling fluid entering (T_(c,0)) and exiting(T_(c,f)) heat exchanger 503. By means of an orifice or magneticflowmeter 515, the flow rate (F) of coolant through the heat exchangeris also continually measured. Subject to adjustment for heat generatedin the oxidation of formaldehyde to formic acid and formic acid tocarbon dioxide, estimation of PMIDA conversion from cumulative ordifferential heat generation may be illustrated as follows:

In determining exothermic heat generated during a batch or continuousoxidation reaction, the following terms may be defined for use inquantifying heat rejected to the external heat exchanger 503 as shown inFIG. 15, assuming a well mixed reactor and approximately steady stateacross the heat exchanger:

{dot over (Q)}=Heat removal rate via heat exchanger, ˜steady stateacross heat exchanger J/secF(t)=cooling water flow, varies with time [=] kg/secT_(c,0)=inlet cooling water temperature [=] ° C.T_(c,f)(t)=outlet cooling water temperature, varies with time [=] ° C.C_(c)=heat capacity of cooling water

$\begin{matrix}\lbrack = \rbrack \\\;\end{matrix}\frac{J}{kg{^\circ}C}$

Assuming steady state across heat exchanger:

{dot over (Q)}=F(t)C _(C)(T _(c,f)(t)−T _(c,0))  (5-1)

R=reaction rate [=] kg/secT(t)=reactor temperature [=] ° C.M=total reaction mass [=] kg, assumed to remain substantially constantM₀=initial mass PMIDA [=] kgM_(f)=final (i.e., residual) PMIDA [=] kgC_(p)=heat capacity of raw mass [=] J/kg° C.T_(R)=reference temperature [=] ° C.ΔH_(R)=heat of reaction [=] J/kg, negative for exothermic reactions

$\begin{matrix}{\frac{d\left( {C_{p}{M\ \left( {T - T_{R}} \right)}} \right)}{dt} = {{- \left( {R\;\Delta\; H_{R}} \right)} - \overset{.}{Q}}} & \left( {5\text{-}2} \right)\end{matrix}$

Q_(E)=cumulative heat removed from system [=] J

Q _(E) =∫{dot over (Q)}dt  (5-3)

Integrate equation 5-2 from some initial time in a batch (e.g., at t=0,when the reactor temperature is T_(o)) through some final time (e.g.,t=f, when the reactor temperature is T_(f)):

$\begin{matrix}{{{C_{p}{M\left( {T_{f}\  - T_{0}} \right)}} = {{{- \Delta}\;{H_{R}\left( {M_{o}\  - M_{f}} \right)}} - Q_{E}}}{or}} & \left( {5\text{-}4} \right) \\{Q_{E} = {{\Delta\;{H_{R}\left( {M_{o}\  - M_{f}} \right)}} - {C_{p}{M\left( {T_{f}\  - T_{0}} \right)}}}} & \left( {5\text{-}5} \right) \\{M_{f} = {M_{o}\  + \frac{{\int\;{F(t){C_{c}\left( {T_{c,f} - T_{c,o}} \right)}{dt}}} + {C_{p}{M\left( {T_{f}\  - T_{0}} \right)}}}{\Delta\; H_{R}}}} & \left( {5\text{-}6} \right)\end{matrix}$

Further as described above with respect to cumulative oxygen consumptionand carbon dioxide generation, the accuracy with which cumulativeexothermic heat generation is used to estimate conversion and/orresidual PMIDA content can be enhanced by combining measurement of heatgeneration with other methods for determining conversion. For example, abase point PMIDA content can be determined analytically from a sampletaken at a relatively high conversion, and heat generation measured fromthe time (or location within a continuous oxidation reactor system) atwhich the base point sample is taken. As in the case of determiningconversion and/or end point from oxygen consumption or CO₂ generation,error in measurement of cumulative heat release, or arising from heatgeneration other than from oxidation of PMIDA, or PMIDA and C₁by-products, is a fraction only of the incremental heat generationduring the final stage of conversion after the base point rather than afraction of the total exothermic heat generated in the conversion of allPMIDA charged to the reactor.

This combined method enjoys the same advantages as the combinedanalytical and oxygen consumption method, or combined analytical and CO₂generation method, as described above. Thus, it is governed by chemicalanalysis up to the high conversion base point during which such analysisis the most reliable, then switches at the base point to cumulative heatgeneration over the final stage of the reaction, during which the lattermethod typically provides accuracy superior to that of chemicalanalysis.

In this combined method, compensation for formation and consumption offormaldehyde and formic acid can be accomplished in the same manner asgenerally described above with respect to the oxygen consumption method,and elaborated below with respect to the heat generation method.

Application of cumulative heat generation after the base point may beillustrated mathematically as follows:

where:CO _(2gen)=cumulative CO₂ generation from t_(o) through t

Gly=N-(phosphonomethyl)glycine

FM=formaldehyde (CH₂O) at time t subsequent to the base pointFA=formic acid at time t subsequent to the base pointFM₀=formaldehyde at the base point as measured analyticallyFA₀=formic acid at base point as measured analyticallyt_(o)=time at the base point analysis (e.g., by FTIR)Rx₁ PMI DA+½ O₂

Gly+CO₂+CH₂ORx₂ FM+½ O₂

FARx₃ FA+½ O₂

CO₂+H₂OFor this reaction system (total molar basis):

${\overset{\_}{{CO}_{2}}}_{gen} = {{2\left( {{PMIDA}_{o} - {PMIDA}} \right)} - \left( {{FM} + {FA}} \right) + \left( {{FM}_{o}\  + \ {FA}_{o}} \right)}$${PMIDA} = {{- \left( \frac{{\overset{\_}{{CO}_{2}}}_{gen} + {FM} + {FA} - {FM}_{o} - {FA}_{o}}{2} \right)} + {PMIDA}_{o}}$$Q_{E} = {{\int_{t_{o}}^{t}\overset{.}{Qdt}} = {\int_{t_{o}}^{t}{{F(t)}{C_{c}\left( {{T_{c,f}(t)} - T_{c,o}} \right)}{dt}}}}$

Equation (5-2) (modified) wherein R_(i) is the rate of reaction forreactions Rx₁, Rx₂ and Rx₃ above and Q_(loss) is heat loss from thereactor system:

$\frac{{dC}_{p}{M\left( {T - T_{R}} \right)}}{dt} = {{- {\sum\limits_{i = 1}^{3}{R_{i}\Delta\; H_{R \times i}}}} - \overset{.}{Q} - {\overset{.}{Q}}_{loss}}$

For any initial time t₀ (but not limited to literal t_(o)=0) the totalheat removed via a heat exchanger through time

$t = {Q_{E} = {{\int_{t_{o}}^{t}\overset{.}{Qdt}} = {\int_{t_{o}}^{t}{{F(t)}{C_{c}\left( {{T_{c,f}(t)} - T_{c,o}} \right)}{dt}}}}}$

Integrate Equation 5-2 (modified):

${\int_{t_{o}}^{t}{\frac{{dC}_{p}{M\left( {T - T_{R}} \right)}}{dt}{dt}}} = {{- {\int_{t_{o}}^{t}{\left( {\sum\limits_{t_{o}}{R_{i}\Delta\; H_{R \times i}}} \right){dt}}}} - {\int_{t_{o}}^{t}\overset{.}{Qdt}} - {\int_{t_{o}}^{t}\overset{.}{Q_{loss}{dt}}}}$

At high conversion, the left side of this equation can be assumedapproximately zero because at this point in the reaction, the reactortemperature is typically well-controlled and approximately constant.Moreover, in the last term on the right side of the equation, Q_(loss)can be assumed approximately a constant for a given reactor system.

${Q_{E} + {{\overset{.}{Q}}_{loss}\left( {t - t_{o}} \right)}} = {- {\int_{t_{o}}^{t}{\left( {\sum\limits_{t_{o}}{R_{i}\Delta\; H_{Rxi}}} \right){dt}}}}$

For the given reaction system (simplified with three reactions) thereactions can be rewritten:

Rx _(A) =PMIDA+1.5O₂

Gly+2CO₂+H₂O ΔH _(A) =ΔH _(Rx1) +ΔH _(Rx2) +ΔH _(Rx3)

Rx _(B) =PMIDA+1O₂

Gly+CO₂ +FA ΔH _(B) =ΔH _(Rx1) +ΔH _(Rx2)

Rx _(C) =PMIDA+1.5O₂

Gly+CO₂ +FM ΔH _(C) =ΔH _(Rx1)

where:

ΔH_(Rx1)=exothermic heat of reaction for Rx₁

ΔH_(Rx2)=exothermic heat of reaction for Rx₂

ΔH_(Rx3)=exothermic heat of reaction for Rx₃

The extent of each the three reactions may be determined by measuring:

AFM and AFA

Heat=(FM−FM ₀)ΔH _(c)=(FM−FM ₀)ΔH _(Rx1)

Heat_(B)=(FA−FA ₀)ΔH _(b)=(FA−FA ₀)(ΔH _(Rx1) +ΔH _(Rx2))

Heat_(A)=

MIDA₀ −PMIDA)−(FM−FM ₀)−(FA−

(ΔH _(Rx1) +ΔH _(Rx2) +ΔH _(Rx3))

Therefore,

Q _(E) +{dot over (Q)} _(loss)(t−t _(o))=−(Heat_(A)+Heat_(B)+Heat_(C))

Q _(E) +{dot over (Q)} _(loss)(t−t _(o))=(PMIDA−PMIDA ₀)(ΔH _(Rx1) +ΔH_(Rx2) +ΔH _(Rx3))−(FM ₀ −FM)(ΔH _(Rx2) +ΔH _(Rx3))−(FA ₀ −FA)(ΔH_(Rx3))

Based on the relationship set forth above, the residual PMIDA present atany given time subsequent to the base point can be determined andconverted to a concentration. Likewise, those skilled in the art willunderstand that the same principles can be applied to estimateconversion and/or residual PMIDA content at any given location in acontinuous oxidation reactor system downstream of the location fromwhich the base point sample is taken.

A third approach uses reaction kinetics and thereby avoids dualmeasurement (i.e., FTIR+heat release or FTIR+cumulative CO₂) and so thatjust FTIR or HPLC can be used.

$\begin{matrix}{\mspace{79mu}{\frac{dPMIDA}{dt} = {- {k_{R \times 1}\lbrack{PMIDA}\rbrack}}}} & \left( {10\text{-}1} \right) \\{\frac{dFM}{dt} = {{{k_{R \times 1}\lbrack{PMIDA}\rbrack} - {k_{R \times 2}\lbrack{FM}\rbrack}} = {\frac{dPMIDA}{dt} - {k_{R \times 2}\lbrack{FM}\rbrack}}}} & \left( {10\text{-}2} \right) \\{\mspace{79mu}{\frac{dFA}{dt} = {{k_{R \times 2}\lbrack{FM}\rbrack} - {k_{R \times 3}\lbrack{FA}\rbrack}}}} & \left( {10\text{-}3} \right)\end{matrix}$

where:

-   -   k_(Rx1)=rate constant for conversion of PMIDA to glyphosate per        Rx₁    -   k_(RX2)=rate constant for conversion of CH₂O to formic acid per        Rx₂    -   k_(RX3)=rate constant for conversion of formic acid to CO₂ per        Rx₃

As those skilled in the art will understand, the rate constants asderived above actually constitute a composite of the actual rateconstant and certain other variables, including mass transfercoefficients and dissolved oxygen content of the aqueous medium.

After the PMIDA concentration of the oxidation reaction solution hasbeen measured analytically, e.g., by FTIR, at relatively high conversionin the non-zero order regime, e.g., 90% to 95%, the FTIR value is usedto calculate k_(Rx1) per equation 10-1. From the established value ofk_(Rx1) and the formaldehyde material balance based on furtheranalytical data for formaldehyde, the value of k_(Rx2) may be determinedfrom equation 10-2; and from the established value of k_(Rx3) and thematerial balance for formic acid based on further analytical data forformic acid, the value of k_(Rx3) may be determined from equation 10-3.The values of k_(Rx1), k_(RX2) and k_(Rx3) may then be used to calculatea new PMIDA concentration. This iterative calculation is continued whilethe PMIDA concentration can be reliably measured using FTIR. Once theconcentration of PMIDA can no longer be reliably measured using FTIR,the ratio of k_(Rx2)/k_(Rx3) based on the previous calculations can beused to calculate k_(Rx1)[PMIDA] by monitoring the concentration offormaldehyde. The ratio of k_(Rx2)/k_(Rx3) may be modified in subsequentcalculations based on operational experience and historical data, e.g.,based on prior batches, to further refine this approach. This iterativecalculation can be alternatively practiced using PMIDA concentrationmeasured by HPLC.

Since the oxidation of PMIDA may be approximated as first order, theresidual PMIDA content may also be inferred from the residual rate ofreaction, as indicated by the instantaneous residual rate of heatgeneration, at the end of the batch, at the exit of a fixed bed reactor,or under the terminal conditions prevailing in the last of a series ofCSTRs. In this regard, the inferences based on the progressivelydeclining rate of reaction during non-zero order reaction are drawnsubstantially as is described above for the estimation of conversionand/or end point from oxygen consumption or CO₂ generation. Essentially,the residual PMIDA concentration is determinable from the instantaneousrate of heat generation as related to the mass of aqueous reactionmedium contained in or flowing through the reaction zone. This methodcan be calibrated in the manner described above with respect to theoxygen consumption method, i.e., by estimation of the kinetic rateconstant or function thereof from long term or short term historicaldata. Moreover, the estimate of the rate constant can be further updatedin the manner described above, i.e., from the first derivative of theheat generation rate during the non-zero order reaction regime, i.e.,the second derivative of cumulative heat generation. Data for updatingsuch estimate can be generated by measuring the instantaneous rate ofheat generation as a function of time under non-zero order reactionconditions, i.e., at different points in time toward the end of a batchreaction cycle, or at differing residence times under terminalconditions prevailing in the final stage of a continuous reactionsystem. As the catalyst ages and its activity declines, the effect onfirst order rate constants can be periodically re-calibrated by samplingthe final stage of a continuous reaction system, or batch reactor nearthe end of the reaction cycle.

Under approximately first order reaction conditions, the reaction rateis approximately proportional to the product of residual PMIDA contentand dissolved oxygen concentration. Where dissolved oxygen is maintainedat a reasonable stable level, the reaction may be proportional toresidual PMIDA concentration. Thus, where the value of the kinetic rateconstant is known, e.g., from a series of samples taken at discreteintervals of time during the non-zero order reaction period of a recentbatch, a recent series of batches, or during a discrete series ofoperations at differing flow rates in a continuous reaction system,residual PMIDA may also be estimated from the instantaneous rate of heatgeneration that is associated with the oxidation of PMIDA:

${{Continuous}\mspace{14mu}\left( {{steady}\text{-}{state}} \right)\mspace{14mu}{or}\mspace{14mu}{batch}\mspace{14mu}{with}\mspace{14mu}\frac{dT}{dt}} \approx O$Assume: R=k·m(t)

-   -   m(t)=system mass of PMIDA        Substitute into equation 5-2 (C_(p), M, T₀ constant)

${C_{p}M} = {\frac{dT}{dt} - {\Delta\; H_{R}{{km}(t)}} - \overset{.}{Q}}$

Assume

$\frac{dT}{dt} \approx O$

at end or reaction:

$\begin{matrix}{\overset{.}{Q} = {{- \Delta}\; H_{R}{k \cdot m}}} & \left( {6\text{-}1} \right) \\{\overset{.}{m} = \frac{Q}{{- \Delta}\;{H_{R} \cdot k}}} & \left( {6\text{-}2} \right)\end{matrix}$

The value of k is dependent on catalyst activity and, thus tends todecline over time. However, the value of k can be constantly updated bymeasuring both the rate of heat generation and the rate at which therate of heat generation declines during the non-zero order reactionperiod of a batch reaction system. Similar information may be providedby operating at different residual times in the non-zero order zonewithin a continuous reaction system, e.g., the last of a series ofCSTRs, or the zone near the exit of a plug flow reactor:

R=k·m

for batch when

$\begin{matrix}{{\frac{dT}{dt} \sim {O\frac{dm}{dt}}} = {{- {km}} = {+ \frac{\overset{.}{Q}}{\Delta\; H_{R}}}}} & \left( {6\text{-}3} \right) \\{{\frac{d^{2}m}{{dt}^{2}} = {{{- k}\frac{dm}{dt}} = \frac{{dQ}/{dt}}{\Delta\; H_{R}}}}{\frac{{- k}\overset{.}{Q}}{\Delta\; H_{R}}\  = \ \frac{{+ d}{\overset{.}{Q}/{dt}}}{\Delta\; H_{R}}}} & \left( {6\text{-}4} \right) \\{k = {\left( {- \frac{d\overset{.}{Q}}{dt}} \right)\frac{1}{\overset{.}{Q}}}} & \left( {6\text{-}5} \right)\end{matrix}$

An updated estimate of the kinetic rate constant, as obtained, forexample, from a given PMIDA oxidation reaction batch, may be used inestimating residual PMIDA in a subsequent batch using the same catalystmass under the same reaction conditions. In a series of continuousreaction zones, the updated rate constant, as based on sampling orinferred from the rate of decline in the rate of heat generation acrossthe final reaction zone during a select period of operation, may be usedin estimating conversions and controlling residence times and oxygenflow rates during other operations, such as: (i) conversion of PMIDA toglyphosate in the same reactor at a different point in real time; (ii)conversion of PMIDA to glyphosate in the same reactor at a differentresidence time; (iii) conversion of PMIDA to glyphosate in a differentfinal back mixed reaction zone; and combinations thereof. Note that themathematical principles for estimation of conversion and rate constant,as outlined above with respect to heat generation, are equallyapplicable to other methods for determining conversion, including oxygenconsumption, CO₂ generation and various analytical methods.

Heat generation data may be used to project the conversion or reactionend point in the manner described above with respect to FTIR, but moreparticularly as described in connection with the oxygen consumptionmethod. While FTIR provides a direct measure of PMIDA content, theoxygen consumption, carbon dioxide generation and heat generationmethods all yield data which are a function of PMIDA content, but thenature of the function must be separately established. The descriptionabove with regard to oxygen consumption deals with this element of themethod. Further in connection with the estimation or projection ofconversion and end point by heat generation, operational data may alsobe used to estimate the order of reaction and/or the kinetic rateconstant, again essentially in the manner described above with respectto FTIR, as modified according the description set forth above for theoxygen consumption method.

To compensate for oxidation of C₁ compounds in the differential heatgeneration method, residual formaldehyde and formic acid concentrationsunder terminal conditions can also be directly measured or estimatedfrom historical data as described above with regard to the estimation ofPMIDA conversion from oxygen consumption and/or CO₂ generation.Alternatively, as noted above, the measured cumulative heat generationcan be adjusted for the effect of C₁ oxidation by estimating residual C₁content from electrochemical oxidation data for C₁ compounds. Moreover,from historical operating data, the kinetic rate constants for oxidationof formaldehyde and formic acid may be a known or knowable function ofthe kinetic rate constant for oxidation of PMIDA, as each of thesevaries with changing catalyst activity and the oxidation/reductionpotential of the aqueous medium.

Also as in the case of oxygen consumption and/or CO₂ generation, theconversions estimated from cumulative heat generation in a continuousreaction system may be adjusted by any difference in the molar rate atwhich PMIDA is introduced into the reaction system vs. the molar rate atwhich the sum of glyphosate and unreacted PMIDA are withdrawn therefrom.

Preferably, estimation of conversion by heat generation is furtheradjusted for various heat losses from the system, including conductiveheat losses to the environment, sensible heat loss to the vent gas (O₂,CO₂ and any other inerts), and evaporative heat loss reflected by thewater vapor, formaldehyde and formic acid content of the vent gas. Tofurther refine the estimate obtained from cumulative or differentialheat generation data, the estimate of the sum of the effects ofenvironmental heat loss and heat loss to said vent gas from a givenreaction batch in a particular reaction zone is adjusted by comparisonof actual conversion data for a given period of operation to an estimateof conversion that had been computed for a previous batch as produced insaid particular reaction zone. Similarly, the estimate for such lossesfrom a continuous reactor for a particular period of operation can beadjusted by comparison with actual conversion data for the same periodof operation at an earlier time.

Estimation of conversion from heat generation data may be used incombination with either or both of the oxygen consumption and carbondioxide generation methods described hereinabove. As noted, it may alsobe used together with data from electrochemical oxidation, either forestimation of C₁ compounds alone, or for a corroborative estimate ofPMIDA also. As stated above, the various methods of the invention can beused in various combinations to maximize the information obtained andenhance process control schemes. Such combinations may be particularlyuseful where one method is used to monitor the conversion of PMIDA andanother method is used to evaluate the kinetics of the reaction orcompensate for C₁ content. All combinations are permutations of thesemethods are contemplated because, as described above, all of them havethe capability of providing useful data on conversion of PMIDA, kineticrate constants and order of reaction. And all can be used either todetect or to project the end point of the reaction or the properresidence time to achieve a desired conversion.

Multiple combinations of these methods may be suitable for C₁compensation. For example, FTIR or HPLC may provide a determination offormaldehyde and formic acid content sufficient to adjust an estimate ofPMIDA content based on oxygen consumption, heat generation,electrochemical oxidation or carbon dioxide generation. The value ofon-line FTIR for this purpose may be enhanced if the instrument is tunedspecifically to follow the formaldehyde and formic acid concentrations.Moreover, non-dispersive i.r. can also be used to monitor C₁s.Additionally or alternatively, a stoichiometric comparison of cumulativeoxygen consumption with cumulative CO₂ generation may provide a basisfor estimating the fraction of formaldehyde and formic acid that areproduced in the oxidation of PMIDA but not oxidized to CO₂. For thispurpose, it may be useful to monitor the difference in instantaneousoxygen consumption and CO₂ generation, and integrate this differenceover time to estimate the accumulation of C₁s and/or the destructionthereof. In a further alternative, select voltage electrochemicaloxidation may be useful for determination of C₁s while conversion ofPMIDA may be monitored by instantaneous heat generation, cumulative heatgeneration, instantaneous oxygen consumption, cumulative oxygenconsumption, FTIR, HPLC, and/or carbon dioxide generation, optionally incombination with select voltage and/or select current electrochemicaloxidation. Where oxygen consumption and/or carbon dioxide generation areused either as primary methods for monitoring PMIDA consumption, or inan auxiliary role for monitoring C₁s, the composition of the vent gasexiting the liquid phase is preferably monitored as described above withrespect to detection of end point by vent gas O₂ or CO₂ content, e.g.,by directing a sample of the liquid phase to a gas/liquid separator orby use of a probe which determines the content of nascent gas phase inthe liquid phase. Where select voltage electrochemical oxidation is usedto monitor PMIDA, PMIDA and C₁s, or C₁s alone, it may be periodicallycalibrated by assaying the reaction mixture, e.g., by HPLC.

Under certain conditions, time alone may function as a useful measure ofconversion and/or end point, or may be used in combination with any oneor any combination of the other methods described herein for thepurpose. Where batch PMIDA and catalyst charge, or continuous reactorPMIDA and catalyst charge rate, can be precisely measured and reliablycontrolled, temperature, oxygen flow and agitation precisely andconsistently controlled, successive batches or successive operations canbe reasonably controlled based on assay of immediately preceding batchesand measurement of reaction time. In such operations, it may bedesirable to introduce the PMIDA charge from a weigh tank and providepositive shut-off valves or disconnect the charge line from the reactorafter charging is complete. Timed reaction may be an attractivealternative where, e.g., a batch reactor is operated with a dedicatedcatalyst mass, i.e., a catalyst mass that is segregated from othercatalyst masses that are used in the same or other reactors in aglyphosate manufacturing facility. If, for example, two separatecatalyst masses are dedicated to a given reactor so that one is beingused in the oxidation reaction while the other is being recovered byfiltration, the reaction time for each batch (n) can be guided by thereaction time and assay of batch (n−2), after which the reaction timefor batch (n+1) can be guided by reaction time and assay of batch (n−1),and so on. In a carefully controlled system, monitoring of total oxygendelivered to the reactor may be used as an alternative to timing thereaction, or as a cross-check against determination of conversion or endpoint by time alone. Note that, where oxygen input is monitored, it isalso useful to monitor oxygen consumption, so that the oxygenconsumption method described above provides a further cross-checkagainst determination of conversion and/or end point based on timeand/or cumulative oxygen flow.

The various methods of the invention for monitoring PMIDA content can beused in monitoring streams other than the product reaction solutionobtained in a PMIDA oxidation reactor. For example, in response to PMIDAconcentration in crystallization mother liquor streams such as, e.g.,streams 129 and 131 of FIGS. 1 and 2, PMIDA purge streams, such as purge133 of FIGS. 1 and 2, adjustments may be made to the conditions ofoperation of a crystallization process, the purge fraction, or thedivision and allocation of process streams for purposes of allocatingunreacted PMIDA among plural glyphosate products. The methods of theinvention may also be useful, for example, in monitoring the operationof an ion exchange system, and in particular for identifyingbreakthrough of chlorides or PMIDA from an ion exchange column whoseduty is to remove them from a stream also comprising glyphosate asdisclosed hereinabove.

Where the order of the oxidation reactions and the rate constantsthereof, etc. are initially determined by laboratory experimentation,such data may be combined with exothermic heats of reaction, materialbalances, energy balances (including estimates of environmental heatlosses and both sensible and latent heat losses to reaction gases), andother process parameters to generate a mathematical model predictive ofconversion and end point as a function of the temperature, oxygen flow,dissolved oxygen, oxidation/reduction potential, agitation, feedcomposition and other process variables that may be imposed and/ormeasured in the field. Where the model is based on oxygen consumption,carbon dioxide generation, and/or heat consumption, it can andpreferably does account for the impact of the oxidation of formaldehydeto formic acid and formic acid to carbon dioxide on these observedeffects. Such an algorithm may integrate any number of the end point andconversion methods described herein, including not only FTIR, but alsoO₂ consumption, CO₂ generation, vent gas analyses, dissolved oxygen,etc. From a select combination of such data, a virtual reaction modelmay be established from which end points can be projected with greataccuracy. Moreover, such projections may utilize FTIR or othermeasurements that are taken, for example, when 10% or more of PMIDA feedremains unreacted and a pseudo zero order reaction, still prevails,e.g., in the next to last of a series of CSTRs or at a point in a batchcycle when reaction rate remains directly proportional to dissolvedoxygen concentration independently of PMIDA content. A control systemthat is programmed with such a model may function to adjust independentvariables to consistently achieve a target PMIDA concentration in theproduct reaction solution. For example, in a batch reaction system, thepoint at which oxygen flow is ramped down or terminated can be adjustedin response to such a predictive model. In a continuous reaction system,feed rates may be adjusted to alter residence times in order to achievea desired conversion and target exit PMIDA content. In either batch orcontinuous operation, a controller programmed with such an algorithm maybe used to adjust temperature, oxygen flow rates, oxygen pressure,intensity of agitation, etc.

In accordance with the invention, the algorithm may be updated bycomputer evaluation of actual plant operating data, including acombination of actual operating conditions and precise laboratoryanalyses of process samples that are taken routinely in the course ofoperations. Such computer evaluation and adjustment may be directlyprogrammed into the operational end point estimation and control systemto further refine the virtual process model that is used in further andongoing end point projection.

A programmed control scheme that incorporates a virtual model of thereactor, but which may also integrate other considerations such asmarket factors is described below.

Programmed Control Scheme

The present invention contemplates the use of essentially allcombinations and permutations of the various measures that are describedhereinabove for reducing the PMIDA content of a glyphosate product. Theinvention further contemplates the use of one or more of theabove-described schemes for monitoring PMIDA conversion, and identifyinga reaction end point based on residual PMIDA content. In this regard,the programmed control scheme may comprise the programmed end point andoxygen flow control models that are illustrated in FIGS. 10 and 14, butis not limited thereto. FIG. 14 provides closed loop control ofconversion by regulating oxygen flow in response to residual PMIDAcontent. In some instances, this may suffice for satisfactory control.In other instances, it may not be technically feasible or economicallyattractive to achieve a target PMIDA concentration by resort solely toincreased oxygen flow rate, or solely to increased purge, or solely toany other single process control stratagem. Although certain processmodifications such as ion exchange, where justified, may be quitesufficient to achieve any desired PMIDA level, there can still beadvantages in adopting ion exchange in combination with otheroperational variations.

In practicing the various methods of the invention, operationalstability, economic optimization, product and emission specificationand/or other advantages and constraints may be met or achieved by aprogrammed control scheme under which a combination of various measuressuch as increased oxygen flow, purge adjustment, allocation of PMIDAamong plural product forms, ion exchange conditions, process flows,reactor and crystallizer temperatures, reactor and crystallizerpressures, etc., may be monitored and controlled at values which achievea target PMIDA specification in one or more product forms according toan optimal or otherwise desirable operational mode. In this connection,it will be understood that what is referred to as the process controlsystem 403 in FIG. 14 may be programmed to integrate input signals otherthan PMIDA content and oxygen flow and generate output signals otherthan oxygen flow set points. The input signals may include, e.g.,reaction temperatures, oxygen flow, oxygen pressure, catalystconcentration, catalyst age and activity, heat generation dissolvedoxygen, oxygen content of vent gas, CO₂ content of vent gas, ORP andvarious scheduling parameters, in addition to signals from one or moreof the methods described above for monitoring PMIDA conversion and/oridentifying oxidation reaction end points. In accordance with such acontrol scheme, signals conveying the prevailing values of variousparameters and the control set points for the control loops for suchparameters may be transmitted to a programmed controller which, inresponse to these inputs, may generate output signals to adjust thevarious set points according to an algorithm inscribed in controllersoftware. For example, the algorithm may be adapted to achieve a targetPMIDA content in a specified glyphosate product form at minimum cost,and/or at maximum throughput, and/or to meet other productspecifications, and/or to conform to emission standards, etc.

Such a program may be periodically adjusted as necessary to reflectchanges in raw material prices, product demand, production scheduling,environmental conditions, etc.

Although the various methods of the invention have been described abovewith respect to the catalytic oxidation of PMIDA to glyphosate, themethods are effective in other oxidation processes. For example, PMIDAconversion and end point detection can be provided in the oxidation ofPMIDA to N-(phosphonomethyl)iminodiacetic acid-N-oxide by reaction witha peroxide compound in the presence of a metal catalyst as described inU.S. Pat. Nos. 5,043,475, 5,077,431 and/or 5,095,140. Each of thevarious methods described herein can be applied to the peroxideoxidation process. FTIR and HPLC analyses operate on the same principlesas for catalytic oxidation to glyphosate. Reaction material balances foroxygen consumption and CO₂ generation are different but known, as areenergy balances for heat generation. For electrochemical methods, theoxidation voltage for oxidation of PMIDA toN-(phosphonomethyl)iminodiacetic acid-N-oxide may not be known but canbe experimentally determined by methods known to the art.

Glyphosate Product

By implementation of one or more of the process modifications andstratagems as described above, a manufactured glyphosate product may berecovered and removed from the process in a desired form with a PMIDAcontent of less than, 6,000 ppm, 5,000 ppm, 4,000 ppm, 3,000 ppm, 2,000ppm, 1,000 ppm, 600 ppm or even significantly lower. A glyphosateproduct of such low PMIDA level can be produced, for example, in theform of a solid crystalline glyphosate acid, or in the form of anaqueous concentrate of glyphosate salt, such as a potassium orisopropylamine salt having a glyphosate content of at least about 360gpl, a.e., preferably at least about 500 gpl, a.e., more preferably atleast about 600 gpl, a.e.

Glyphosate having a relatively low PMIDA content, e.g., not greater thanabout 0.45 wt. % acid equivalent on a glyphosate, a.e., basis, can beprepared by any of a variety of manufacturing processes. Significantcommercial advantages result from the preparation of glyphosate by aprocess comprising the catalytic oxidation of a PMIDA substrate asdescribed in detail hereinabove. Glyphosate obtained in this manner hasa very low glyphosine content, typically less than about 0.010 wt. %acid equivalent on a glyphosate a.e. basis. It generally has a small butacceptable glycine content, i.e., at least about 0.02 wt. % acidequivalent as also computed on a glyphosate, a.e., basis. PMIDA-derivedglyphosate product may also include small, but acceptable concentrationsof a number of other by-products and impurities. These may include forexample: iminodiacetic acid or salt thereof (IDA) in a concentration ofat least about 0.02 wt. % acid equivalent on a glyphosate, a.e., basis;N-methyl glyphosate or a salt thereof (NMG) in a concentration of atleast about 0.01 wt. % on a glyphosate, a.e., basis; N-formylglyphosateor a salt thereof (NFG) in a concentration of at least about 0.010 wt. %acid equivalent on a glyphosate, a.e., basis; iminobis(methylenephosphonic acid) or a salt thereof(iminobis) in aconcentration of at least about 0.010 wt. % acid equivalent on aglyphosate, a.e., basis; and N-methylaminomethylphosphonic acid (MAMPA)or a salt thereof in a concentration of at least about 0.010 wt. % acidequivalent on a glyphosate, a.e., basis.

These relative proportions generally apply regardless of the form of theglyphosate product, i.e., regardless of whether it is in the form ofsolid state glyphosate acid or a concentrated aqueous liquid solutioncomprising a glyphosate salt such as, for example, a potassium,isopropylamine, monoammonium or diammonium salt. Preferred aqueousconcentrates comprise at least about 360 grams per liter glyphosate onan acid equivalent basis, with proportionate minor concentrations of thecommon by-products and impurities as listed above.

Further detailed limits and ranges for IDA, NMG, AMPA, NFG, iminobis,and MAMPA are set out below. All are expressed on an acid equivalentbasis relative to glyphosate, a.e.

More typically, the IDA content may be between about 0.02 wt. % andabout 1.5 wt. %, e.g., between about 0.05 wt. % and about 1.0 wt. %, ona glyphosate a.e. basis. Preferably, the IDA content is not greater thanabout 0.58 wt. %, not greater than about 0.55 wt. %, or not greater thanabout 0.50 wt. % on the same basis. In most operations, the productobtained has an IDA content between about 0.1 and about 0.58 wt. %,between about 0.1 and about 0.55 wt. %, between about 0.02 and about0.55 wt. %, or between about 0.1 and about 0.50 wt. %.

Generally, the NMG content is between about 0.02 and about 1.5 wt. %,for example, between about 0.02 and about 1.0 wt. %, or between about0.070 and about 1 wt. % on a glyphosate, a.e., basis. Preferably, theNMG content is not greater than about 0.55 wt. % or not greater thanabout 0.50 wt. %.

The glyphosate product also typically contains aminomethylphosphonicacid or a salt thereof (AMPA) in a concentration that may beincrementally higher than that of glyphosate products which haverelatively higher residual PMIDA content. For example, the AMPA contentmay range between about 0.15 and about 2 wt. %, more typically betweenabout 0.2 and about 1.5 wt. % aminomethylphosphonic acid or a saltthereof on a glyphosate, a.e., basis. In most instances, the AMPAcontent is at least about 0.30 wt. % on the same basis.

The NFG content is ordinarily between about 0.01 and about 1.5 wt. %,e.g., between about 0.03 and about 1.0 wt. %, more typically betweenabout 0.010 and about 0.70 wt. % on a glyphosate, a.e., basis. It isgenerally preferred that the NFG content be not greater than about 0.70wt. %, not greater than about 0.60 wt. %, not greater than about 0.50wt. %, not greater than about 0.40 wt. %, or not greater than about 0.30wt. % on the same basis.

Typically the iminobis content of the glyphosate product is betweenabout 0.1 and about 1.5 wt. %, e.g., between about 0.2 and about 1.0 wt.% on a glyphosate, a.e., basis. Preferably, the iminobis content is notgreater than about 0.8 wt. % iminobis(methylenephosphonic acid),normally between about 0.2 and about 0.8 wt. % on the same basis.

The MAMPA content is ordinarily between 0.1 about and about 2 wt. %,e.g., between 0.15 about and about 1.0 wt. % on a glyphosate, a.e.,basis. Most typically, the MAMPA content is at least about 0.25 wt. %MAMPA on the same basis. Most PMIDA-derived product comprises betweenabout 0.25 and about 0.6 wt. % MAMPA.

Although the typical levels of these various impurities and by-productsare inconsequential so far as the function, use and handling of theglyphosate product is concerned, they serve as markers which distinguisha product produced by catalytic oxidation of PMIDA from glyphosateproduct as produced by other processes. The presence of such impuritiesand by-products in the upper portions of the above described ranges havesome measurable impact on manufacturing process yields, and thus onproduct manufacturing cost.

To provide a reliable commercial source of glyphosate having arelatively low residual PMIDA content, it is necessary to either operatethe manufacturing process on a sustained basis to consistently produceglyphosate product of low PMIDA content, or to segregate product fromdesignated operations in order to accumulate commercial quantities oflow PMIDA product.

Although glyphosate products having a low PMIDA content have beenincidentally produced on a transient basis during startup of amanufacturing facility for the catalytic oxidation of PMIDA toglyphosate, or in operation well below rated capacity, the processes ofthe prior art have not been effective for the preparation of a low PMIDAglyphosate product on a continuing basis during steady state operationsat or near capacity. Thus, each of the various glyphosate products ofthe invention encompasses a lot, run, shipment, segregate, campaign orsupply of glyphosate product as produced by a process capable ofmaintaining a low PMIDA content on a continuing basis. According to thepresent invention, such a lot, run, shipment, campaign, segregate orsupply comprises a quantity of solid state glyphosate acid, orconcentrated aqueous solution of glyphosate salt, comprising at least1500 metric tons, preferably at least about 3000 metric tons, glyphosateon a glyphosate a.e. basis.

For purposes of this invention, a “lot” may be considered a designatedquantity of glyphosate product that is produced under substantiallyconsistent process conditions in a particular manufacturing facilityduring a defined period of operations or over a designated period oftime. Production of the lot may be interrupted for production of otherglyphosate product or non-glyphosate product, or purge of impuritiesfrom the process, but not otherwise by catalyst replacement, turnaroundor startup operations. Glyphosate may be produced according to variousdifferent processes, some of which (e.g., a process comprising theaqueous phase catalytic oxidation of PMIDA) can be conducted in either abatch or continuous mode in the oxidation step and/or or in the recoveryof glyphosate by crystallization thereof from an aqueous medium. Withreference to a process comprising a batch reaction and/or batchglyphosate crystallization operation, it is understood that a lot maycomprise the product of a plurality of batches.

A “run” is quantity of glyphosate product made in a particularmanufacturing facility in continuing or consecutive operations over adesignated period without interruption for maintenance, catalystreplacement, or catalyst loading. It may include both startup and steadystate operations. With reference to a batch reaction and/or batchglyphosate crystallization operation, it is understood that a run maycomprise the product of a plurality of batches.

A “campaign” is a series of runs conducted over an identifiable periodof time during which the runs may be interrupted by other runs not partof the campaign or by purge of impurities, or for maintenance, but notby turnaround or catalyst replacement. No more than one of the runs mayinclude startup operations; provided, however, that more than one of theruns may comprise operation at a rate more than 30% below establishedcapacity. Compare the description of startup operations as set outhereinbelow.

A “shipment” is a commercial quantity of glyphosate product transportedto a particular customer or user in either a single unit, singlecombination of units, consecutive units, or consecutive combinations ofunits without interruption by transport of a commercial quantity of aglyphosate product of materially different average PMIDA content on aglyphosate, a.e., basis to the same user or customer. A materiallydifferent PMIDA content is PMIDA content that is either more than 0.15wt. % higher than the average PMIDA content of the shipment on aglyphosate, a.e., basis, more than 35% higher than the average PMIDAcontent of the shipment on a PMIDA basis, or is above 4500 ppm on aglyphosate a.e. basis.

A “supply” is a series of shipments that may be interrupted by othershipments of glyphosate product to other customers or users.

A “segregate” is a quantity of glyphosate product that is isolated fromother glyphosate product produced in the same manufacturing facilityover the same period of time (i.e., the time during which the segregateis produced). The segregate may be produced in different runs, and maybe allocated among different shipments or different supplies.

Startup operations are operations that are conducted in a manufacturingfacility in which glyphosate product has not previously been produced,or directly following interruption of the production of glyphosateproduct and removal of a substantial fraction of the inventory ofprocess liquids contained in process equipment, with the effect oflowering the total inventory of by-products and impurities in theprocess facility by at least 25 wt. %. Impurities and by-productsinclude PMIDA, IDA, AMPA, NMG, NFG, iminobis(methylenephosphonic acid),MAMPA, formic acid, NMIDA, glycine and glyphosine. For purposes of thisinvention, operations at a rate that is more than 30% below currentlyestablished capacity of a manufacturing facility is also deemed withinthe ambit of startup operations.

Following are Examples presented to illustrate the present invention andare not intended to limit the scope of this invention. The examples willpermit better understanding of the invention and perception of itsadvantages and certain variations of execution.

Example 1 Use of Dual Probe Electrode in Electrochemical OxidationMethod for End Point Determination in Catalytic Batch Oxidation of PMIDAto Glyphosate

To a 1 liter autoclave pressure vessel was charged 11.4 grams of 99%purity PMIDA. 1.4 grams of fresh activated carbon catalyst was added,along with 420 grams of water. The pressure vessel was closed, andagitation at 300 rpm was started. The dual probe electrode was mountedin the bottom of this vessel. Oxygen gas was introduced subsurface intothe vessel, and the pressure was allowed to rise to 60 psig. Excessoxygen and other off gases were allowed to vent to atmosphere. Thereaction mass self heated to 90° C., and was maintained at thistemperature by heat removal through a chilled water cooling coil. Theprogress of the reaction was followed by tracking the voltage response.After about 30 minutes, the voltage began to rise, indicating thedisappearance of PMIDA. After a voltage rise of 0.2 volts, the reactionwas stopped. The glyphosate solution was pumped out of the autoclavethrough a fritted filter, leaving the carbon catalyst behind. Thefiltrate solution was cooled, and the glyphosate allowed to crystallize.Residual PMIDA was determined to be very low.

Example 2 Use of Cumulative Heat and CO₂ Gas Evolution to Track theProgress of the Reaction of PMIDA to Glyphosate

The referenced chemical process takes place in two steps. In the first,PMIDA is oxidized to PMIDA N-oxide with hydrogen peroxide in thepresence of a tungsten catalyst. This PMIDA N-oxide intermediatematerial is isolated, and then catalytically decomposed to glyphosateaccompanied by an equimolar generation of CO₂. This catalyticdecomposition is highly exothermic, and is controlled by the rate ofvanadium sulfate catalyst addition. If the catalyst addition is toofast, the reaction can overheat and switch to an uncontrolled thermaldecomposition. The rate of CO₂ evolution is used to track the reactionrate and control catalyst addition. The cumulative heat generation inthis step is used to determine when the reaction reaches 90% completion,at which point the catalyst addition is stopped.

Into a 16000 liter vessel was added 2100 liters of water, 1600 kg ofPMIDA and 4 kg of Na₂WO₄ catalyst. This slurry was heated to 60° C. 394liters of H₂O₂ was slowly added to form the PMIDA N-oxide. Excessperoxide was destroyed with addition of sodium metabisulfite. Theintermediate (PMIDA N-oxide) material slurry was pumped to a 12000 litervessel, where 3 kg of 33% VOSO₄ was slowly added. CO₂ evolution wasclosely monitored to track reaction rate, and cumulative heat was usedto track the % completion of the reaction. If CO₂ evolution exceeded2000 liters per minute, the rate of catalyst addition was slowed orstopped until the CO₂ evolution rate dropped below 2000 liters perminutes. Reaction heat was totalized until about 290,000 kCal of heatwas removed (90% completion). At this point, the catalyst addition wasstopped, and the reaction slurry cooled. Solid glyphosate was separatedby centrifugation. PMIDA and PMIDA N-oxide levels were found to be verylow.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above description withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

When introducing elements of the present invention or the preferredembodiments thereof, the articles “a”, “an”, “the”, and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including,” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

1.-6. (canceled)
 7. A method for monitoring or detecting the conversionof N-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycineor another intermediate for N-(phosphonomethyl)glycine in the course ofthe catalytic oxidation of N-(phosphonomethyl)iminodiacetic acid in anaqueous medium, the method comprising: introducing an aqueous mediumcontaining N-(phosphonomethyl)iminodiacetic acid into an oxidationreaction zone; contacting N-(phosphonomethyl)iminodiacetic acid withmolecular oxygen in said aqueous medium within said oxidation reactionzone in the presence of a catalyst for the oxidation, thereby effectingoxidation of N-(phosphonomethyl)iminodiacetic acid and producingN-(phosphonomethyl)glycine or said another intermediate; measuring theconsumption of molecular oxygen in said reaction zone; and estimatingthe proportion of N-(phosphonomethyl)iminodiacetic acid that has beenconverted to N-(phosphonomethyl)glycine or said another intermediate insaid reaction zone, said estimating comprising comparing the oxygenconsumed in said reaction zone with the mass ofN-(phosphonomethyl)iminodiacetic acid charged to the reaction zone andthe unit oxygen consumption required for the oxidation ofN-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine orsaid other N-(phosphonomethyl)glycine intermediate.
 8. (canceled)
 9. Amethod for monitoring or detecting the conversion ofN-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine oranother intermediate for N-(phosphonomethyl)glycine in the course of thecatalytic oxidation of N-(phosphonomethyl)iminodiacetic acid in anaqueous medium, the method comprising: introducing an aqueous mediumcontaining N-(phosphonomethyl)iminodiacetic acid into an oxidationreaction zone; contacting N-(phosphonomethyl)iminodiacetic acid withmolecular oxygen in said aqueous medium within said oxidation reactionzone in the presence of a catalyst for the oxidation, thereby effectingoxidation of N-(phosphonomethyl)iminodiacetic acid and producingN-(phosphonomethyl)glycine or said another intermediate; continually orrepetitively measuring the oxygen that is consumed in said reactionzone; monitoring the instantaneous rate of oxygen consumption in thereaction zone in the conversion of N-(phosphonomethyl)iminodiacetic acidto N-(phosphonomethyl)glycine or said another intermediate; andestimating the residual concentration ofN-(phosphonomethyl)iminodiacetic acid in said aqueous medium within saidreaction zone, said estimating comprising comparing said rate of oxygenconsumption with the mass of aqueous medium containingN-(phosphonomethyl)iminodiacetic acid that is charged to the reactionzone or a function thereof. 10.-11. (canceled)
 12. A method formonitoring or detecting the conversion ofN-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine oranother intermediate for N-(phosphonomethyl)glycine in the course of thecatalytic oxidation of N-(phosphonomethyl)iminodiacetic acid by reactionwith molecular oxygen in an aqueous medium, the method comprising:monitoring the O₂ content of the vent gas from, or O₂ utilization withinsaid aqueous medium; and determining the conversion or identifying anend point of the reaction based on the O₂ content of the vent gas or theO₂ utilization within said aqueous medium. 13.-14. (canceled)
 15. Amethod for monitoring or detecting the conversion ofN-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine oranother intermediate for N-(phosphonomethyl)glycine in the course of thecatalytic oxidation of N-(phosphonomethyl)iminodiacetic acid in anaqueous medium, the method comprising: monitoring the dissolved oxygencontent of said aqueous medium; and determining the conversion oridentifying an end point of the reaction based on the dissolved oxygencontent of said medium.
 16. (canceled)
 17. The method of claim 7comprising a plurality of measurements of the oxygen consumed in saidreaction zone.
 18. The method of claim 7 wherein said reaction zone iscontained within a batch oxidation reactor, and estimating saidconversion comprises comparing the cumulative consumption of oxygenduring the reaction with the initial charge ofN-(phosphonomethyl)iminodiacetic acid introduced into said reactionzone.
 19. The method of claim 7 wherein said reaction zone is containedwithin a continuous reactor, and estimating said conversion comprisescomparing the consumption of oxygen in said reaction zone over a selectperiod of time vs. the quantity of N-(phosphonomethyl)iminodiacetic acidintroduced into said reaction zone over said period of time.
 20. Themethod of claim 7 further comprising measuring the instantaneous rate ofoxygen consumption as a function of time at high conversion andadjusting the estimated conversion to account for said measured change;wherein samples are taken during a period of non-zero order reaction toprovide a basis for estimating the effective kinetic rate constant or afunction thereof during the course of continuing reaction at highconversion; and said kinetic rate constant or function thereof is usedin estimating residual N-(phosphonomethyl)iminodiacetic acid content ofthe aqueous reaction medium in a subsequent batch from the instantaneousrate of oxygen consumption during the non-zero order portion of suchsubsequent batch.
 21. The method of claim 9 comprising monitoring theinstantaneous rate of oxygen consumption during non-zero orderconversion of N-(phosphonomethyl)iminodiacetic acid toN-(phosphonomethyl)glycine or said another intermediate.
 22. The methodof claim 9 wherein samples are taken during a period of non-zero orderreaction to provide a basis for estimating the effective kinetic rateconstant or a function thereof during the course of continuing reactionat high conversion; and said kinetic rate constant or function thereofis used in estimating residual N-(phosphonomethyl)iminodiacetic acidcontent of the aqueous reaction medium from the instantaneous rate ofoxygen consumption in subsequent operation.
 23. The method of claim 22wherein said estimated kinetic rate constant is adjusted based on therate of decline of the rate of oxygen consumption as a function of timeduring non-zero order oxidation reaction.
 24. The method of claim 9wherein said reaction zone is contained within a continuous reactor, andestimating said conversion comprises comparing the consumption of oxygenin said reaction zone over a select period of time vs. the quantity ofN-(phosphonomethyl)iminodiacetic acid introduced into said reaction zoneover said period of time.
 25. The method of claim 12 wherein conversionof N-(phosphonomethyl)iminodiacetic acid and/or an end point of thereaction is indicated by the instantaneous concentration of O₂ in saidvent gas or an instantaneous estimate of O₂ utilization.
 26. The methodof claim 12 wherein conversion or end point is determined as a functionof instantaneous concentration of O₂ in the vent gas or instantaneousestimate of O₂ utilization as attained during a stage of batch reaction,or in a continuous reaction zone, after anN-(phosphonomethyl)iminodiacetic acid conversion of 95% has beenattained.
 27. The method of claim 12 comprising monitoring the rate ofchange of the O₂ content of said vent gas or the O₂ utilization as afunction of batch reaction time or continuous oxidation residence time.28. The method of claim 27 wherein a desired conversion or end point isidentified based on historical data correlating the rate of change inthe O₂ content of the vent gas or the O₂ utilization with the conversionor residual N-(phosphonomethyl)iminodiacetic acid content of the aqueousliquid medium.
 29. The method of claim 15 wherein conversion ofN-(phosphonomethyl)iminodiacetic acid and/or an end point of thereaction is indicated by the absolute concentration of dissolved oxygenin said medium.
 30. The method of claim 29 wherein conversion or endpoint is determined as a function of absolute dissolved oxygen contentof the aqueous medium as attained in a batch reaction or in a continuousreaction zone during non-zero order conversion ofN-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine orsaid another intermediate.
 31. The method of claim 15 wherein a desiredconversion or end point is identified based on historical datacorrelating the dissolved oxygen in the aqueous medium with theconversion or residual N-(phosphonomethyl)iminodiacetic acid content ofthe aqueous liquid medium.
 32. The method of claim 15 comprisingmonitoring the rate of change of the dissolved oxygen content of saidaqueous medium as a function of batch reaction time or continuousoxidation residence time.